Question. Maximum no of unsuccessful attempts, a thief can do in order to break a number locking system of 4 digits.
Options:
(A) 9988
(B) 9999
(C) 9799
(D) 9685
(E) 6545
Answer. (B)
What step-by-step reasoning justifies that answer?

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Reasoning: From 0 to 9999 there are 10000 four digit numbers
So maximum number of unsuccessful attempts = 10000 - 1 = 9999