SNEED, Senior Circuit Judge:
Alvin E. Johnson appeals following his conviction by jury on September 14, 1988, of two counts of knowingly and intentionally distributing cocaine base (“crack”) in violation of 21 U.S.C. § 841(a)(1) and (b)(1)(B). The lower court sentenced him to two terms of confinement of 78 months to run concurrently. We affirm.
I. FACTS AND PROCEEDINGS BELOW
On July 5, 1988, a confidential informant working with the Kansas City, Missouri Police Department agreed to introduce undercover officer Phillip Craig Inman to Johnson for the purpose of arranging a drug sale. That afternoon, the officer and the informant went to the defendant’s house, where Inman asked Johnson for one-half ounce of crack cocaine, which Johnson agreed to sell. While they were waiting for delivery of the drugs, the officer mentioned that he might be willing to purchase a kilogram at a later time. Johnson replied that his source could supply any quantity desired.
On July 7, 1989, when Inman telephoned the appellant, Johnson at his own initiative asked whether Inman was still interested in purchasing narcotics. They discussed delivery terms and various prices for varying amounts of cocaine, but set no date for the sale. On July 12, 1989, Inman telephoned and asked Johnson if he could go to Johnson’s house to buy one-half ounce of cocaine. When Inman arrived, they again discussed a future larger deal, and settled on a one-pound amount for $15,000. After Johnson’s supplier arrived with the cocaine, Johnson consummated the sale of 12.1 grams of crack for $700.
On July 14, 1979, Inman again telephoned Johnson and discussed purchasing one pound of cocaine for $15,000. Inman requested all crack, but Johnson suggested an even split between crack and powder cocaine. Johnson explained that Inman could sell the drugs more easily if he also had a supply of powder cocaine, because many people prefer it to crack. They arranged to meet the following day at 7:00 p.m. at the Antin House of Video Store to complete the deal. Inman arrived at the appointed hour and met Johnson who, after contacting his source, told Inman that he could get one pound of powder cocaine. Inman agreed to make the purchase, but broke off negotiations and left the scene after waiting nearly an hour for Johnson’s source to arrive with the drugs. The police arrested Johnson on July 15, 1988, and the grand jury returned a two-count indictment three days later. The trial commenced on September 13, 1988, and the jury found the defendant guilty of both counts the following day.
During the government’s opening statement at the trial, the defendant for the first time requested the identity of the informant. After the government objected and stated that the informant would not be called as a witness, the court refused to order disclosure. After the examination of each witness, the judge asked the jury if it had any questions for the witness. Individual jurors responded orally to the judge, who then submitted appropriate questions to the witness. Defendant did not object. One witness was asked a single question and the defendant was asked five, only one of which the judge prohibited.
II. JURISDICTION
This court has jurisdiction under 28 U.S.C. § 1291. The Notice of Appeal was timely filed under Fed.R.App.P. 4(b).
III. ANALYSIS
A. Insufficiency of the evidence.
Appellant contends that the evidence was insufficient to support his conviction, but he assigns no particular point of error to the lower court’s judgment. This court can reverse only if “a reasonable jury could not have found guilt beyond a reasonable doubt.” United States v. Meeks, 857 F.2d 1201, 1204 (8th Cir.1988). Thus, the verdict must be sustained if supported by sufficient evidence. United States v. Randle, 815 F.2d 505, 508 (8th Cir.1987). At trial, appellant admitted to making two distributions of crack cocaine and negotiating for a third. The undercover officer testified that these transactions occurred. We conclude that a reasonable jury easily could return a guilty verdict on these facts.
B. Improper Fed.R.Evid. 404(b) Evidence.
At trial, the judge overruled appellant’s objection to evidence pertaining to the attempted distribution on July 15, 1988, of one pound of cocaine three days after the second transaction. Appellant contends that the judge erred. Federal Rule of Evidence 404(b) permits admission of “other crimes” evidence “unless it tends to prove only the defendant’s criminal disposition.” United States v. Kandiel, 865 F.2d 967, 972 (8th Cir.1989). Because 404(b) is a rule of inclusion, rather than exclusion, this court will not disturb the trial court’s discretion absent a showing by the defendant that the proof “ ‘clearly had no bearing upon any of the issues involved.’ ” United States v. Galyen, 798 F.2d 331, 332 (8th Cir.1986) (quoting United States v. Estabrook, 774 F.2d 284, 287 (8th Cir.1985)); United States v. DeLuna, 763 F.2d 897, 912-13 (8th Cir.), cert. denied, 474 U.S. 980, 106 S.Ct. 382, 88 L.Ed.2d 336 (1985). In this case, the evidence was relevant to the defendant’s knowledge, intent, and predisposition to distribute cocaine base. We conclude that the lower court did not abuse its discretion. The admission of evidence regarding the attempted third transaction was sufficiently probative to outweigh any potential prejudice. See also United States v. Burkett, 821 F.2d 1306, 1309-10 (8th Cir.1987).
C. Failure to Identify the Informant.
As a third point of error, appellant alleges that disclosure of the government’s confidential informant should have been ordered, even though he knew his identity. This contention is without merit. There is no absolute rule requiring disclosure, McCray v. Illinois, 386 U.S. 300, 311, 87 S.Ct. 1056, 1062, 18 L.Ed.2d 62, reh’g denied, 386 U.S. 1042, 87 S.Ct. 1474, 18 L.Ed.2d 616 (1967), and disclosure is not required where, as here, the defendant already knows the informant’s identity. United States v. Parker, 836 F.2d 1080, 1083 (8th Cir.1987), cert. denied, 486 U.S. 1025, 108 S.Ct. 2002, 100 L.Ed.2d 233 (1988). The defendant has the burden to show that disclosure of the informant’s identity would be material and helpful, see id., a burden the appellant did not carry here. Accordingly, the trial judge properly exercised his discretion to withhold the disclosure of the informant’s identity.
D. Refusal to Instruct on “Knowingly”.
Appellant next assigns as error the trial court’s refusal to submit a jury instruction on the definition of “knowingly,” and cites United States v. Anderton, 629 F.2d 1044, 1049 (5th Cir.1980), appeal after remand, 679 F.2d 1199 (1982). In Ander-ton, however, the court reversed and remanded for a new trial because the court failed to instruct the jury on the word “agent,” a term the court noted “carrped] a rather esoteric meaning.” Id. at 1049. This court has concluded that “knowledge” is “a word of common usage and thus within the ordinary understanding of a juror.” United States v. Smith, 635 F.2d 716, 720 (8th Cir.1980). We have no trouble reaching the same conclusion for the word “knowingly.” Defining this term would not aid the jury and might lead to unnecessary confusion. We thus affirm the lower court’s refusal to instruct the jury on the meaning of “knowingly.”
E. Refusal to Instruct on the Entrapment Defense.
Next is the argument that the lower court erred by refusing to submit an entrapment instruction to the jury. Cited is a Fifth Circuit decision, United States v. Timberlake, 559 F.2d 1375 (5th Cir.1977). In Timberlake, the court reversed because the trial court refused to charge the jury on entrapment after counsel had made it the fulcrum of the defense. In Timberlake it was observed: “ ‘Entrapment occurs only when criminal conduct is the product of the creative activity of government officials.’ ” 559 F.2d at 1378 (quoting United States v. Groessel, 440 F.2d 602, 605 (5th Cir.), cert. denied, 403 U.S. 933, 91 S.Ct. 2263, 29 L.Ed.2d 713 (1971)).
This circuit has determined that a lower court may refuse to submit an entrapment instruction when the defendant exhibited a disposition to engage in the criminal activity with which he was charged. United States v. Irving, 827 F.2d 390, 392 (8th Cir.1987). Appellant testified that he sold the crack to obtain money, thus evidencing a predisposition to commit the crime. Where the government merely provides an opportunity, the lower court may properly reject a requested entrapment instruction.
F. Juror Questioning of Witnesses.
Appellant’s sixth claim of error is that the trial judge incorrectly permitted jurors to ask questions of the witnesses. Counsel for appellant concedes the absence of any authority supporting his position. Nor did he lodge a timely objection at trial. The standard of review for an objection not raised at trial is plain error. Fed.R.Crim.P. 52(b); United States v. Land, 877 F.2d 17, 19 (8th Cir.), cert. denied, — U.S. -, 110 S.Ct. 243, 107 L.Ed.2d 194 (1989). Moreover, this court has found that permitting jurors to question witnesses is not itself plainly erroneous. See Land, supra, at 19 (following DeBenedetto v. Goodyear Tire & Rubber Co., 754 F.2d 512, 516 (4th Cir.1985) (expressing reservations of juror questioning but finding no error) and United States v. Polowichak, 783 F.2d 410, 413 (4th Cir.1986) (same)). In light of this authority, we express no opinion on the appearance and propriety of juror questioning in general and conclude that the lower court did not plainly err in permitting such questions.
G. Constitutionality of the Sentencing Act.
Finally, appellant asserts that his sentencing under the Sentencing Reform Act of 1984, 18 U.S.C. § 3551 et seq., 28 U.S.C. § 991 et seq., should be reversed because that Act is unconstitutional. He fails to ground this allegation on any particular hardship to him as applied to or on any particular provision of the Constitution. Following Mistretta v. United States, — U.S. -, 109 S.Ct. 647, 102 L.Ed.2d 714 (1989), which upheld the constitutionality of the Act against a separation of powers attack, this court ruled that, on its face, the Act did not violate the due process clause. See United States v. Brittman, 872 F.2d 827, 828-29 (8th Cir.), cert. denied, — U.S. -, 110 S.Ct. 184, 107 L.Ed.2d 140 (1989) (rejecting facial challenge but noting that due process may prohibit certain applications of the Act). Absent a specific challenge, therefore, the district court’s application of the Sentencing Act is affirmed.
AFFIRMED.
. The judge did not permit a question concerning recovery of the government’s money as outside the scope of the defendant’s knowledge.