In paragraph 1, "They were linked by telephones", "they" refers to ______.

A．Many computers were attacked by a virus in the U. S.
B．Many computers attacked by a virus in the U.S. lost their information.
C．A computer virus can harm human body.
D．A computer virus can help the computer owner do many things.

A．is the same as a biological virus
B．is also an organism
C．is a series of electronic commands
D．is one of biological viruses