On October 12, 1987, some Chinese scientists were working at the computers to look for information they needed. Suddenly they saw a lot of very bright spots( )crossing the computers' screens. At the same time the computers were working much slower. To find out what was happening they stopped their work to check some parts of the computers. To their horror , they found out that most of their stored information was got rid of( ) by computer viruses! Obviously all these computers had been infected( ) by computer viruses. It is said that the computer viruses were made by a group of young man fond of playing tricks. They all had excellent education. They created the viruses just to show their intelligence. These kinds of computer viruses are named Jerusalem(  )Viruses. These viruses can stay in computers for a long time. When the time comes they will attack the computers by lowering the function , damaging their normal programs or even getting rid of all the information. We now come to know that Jerusalem Viruses often attack computers on Fridays and that they are spreading  to a list of computers. Among the countries that suffering computer viruses are Britain, Australia, Switzerland and the U.S. But till now, how to get rid of the terrible remains a problem. According to the passage, computer viruses seem to   _  .
have been in nature for years be difficult to get rid of at present exist  in any computers be able to be got rid of in the near future