2. Passage 20 Clifford Stoll ran a computer system in Berkelely， California. As a system administrator， he kept a close eye on the records. One day， Stoll found something strange in the payment record. There was a 75 cents mistake. One of the thousands of users had used the system for 75 worth of time and not paid for it. It might sound like very little money to worry about. But that 75 cents was the first clue Stoll had to a much bigger problem — that a hacker(黑客)had broken into Stoll‘s computer system. Who was it? Stoll spent the next year trying to find out. During that time the hacker used the system in Berkelely as a starting point to break into military computer systems all over the United States. Stoll had to keep track of the hacker‘s activities on the computer without the hacker‘s knowing someone was watching him. Stoll even made up huge files of false information so the hacker would have lots to read. When the hacker spent long time reading， the telephone company was able to trace the lines. After a year， the hacker was traced back to his computer — in Germany. He was later caught by the German police. The expression ―keep a close eye on‖ in paragraph 1 most probably means ( ).