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

A．computers
B．government agencies
C．private companies
D．universities

Recently, several thousand computers in the United States were attacked by a virus. The computers were slowed or shut down. But no information was lost. A computer virus is similar to a biological virus--an organism (生物体) that can harm the human body. A computer virus is a series of electronic commands that can harm a computer or the information in the computer. It infects the device (设备) secretly. It tells the computer to do something the computer’s owner does not want it to do. For example, a virus could enter a bank’s computer system. It might tell the system to destroy all information about money belonging to everyone with the first name United States several months ago. The virus was created by a university student studying computer science. The computers affected by the virus were in major universities, government agencies (政府机关) and private (私人) companies. They were part of a United commands in a computer at his school. The computer sent the commands to other computers through the linked telephone lines. The virus told each computer to make many copies of itself. Within a few minutes, all the computers’ power was being used to make copies of the virus. The computer could do no other work. It finally slowed down greatly, or simply stopped working. Computer experts spent many days trying to destroy the virus in the computer system. The experts agree the virus could have been much worse. They say it could have sent orders to destroy huge amounts of electronic information. Many experts believe, the recent computer virus showed the need for better computer security (安全). But that is a problem. The affected system provides a free exchange (交换) of ideas and information among universities, private companies and government offices. Increasing security too much would destroy this exchange. It would slow progress on many important research projects.