In a little-known part of the counter-terrorism world, one of the most effective detection systems is a 600-pound animal【C1】____

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问题     In a little-known part of the counter-terrorism world, one of the most effective detection systems is a 600-pound animal【C1】________works for about 20 pounds of fish a day.
    Since the 1960s,  the United States and a handful of other countries have trained dolphins and sea lions to detect sea mines and swimmers,【C2】________( recover) inert torpedoes (鱼雷) and to test objects used in Naval exercises.
    Program officials estimate that the sea lions in the Marine Mammal Program【C3】________(recover) millions of dollars of U. S. Naval torpedoes and instrumentation【C4】________(drop) on the sea floor.
    The U.S. Navy kept its Marine Mammal Program a secret until the 1990s, and this spring CNN became one of only a handful of media outlets to see firsthand 【C5】________the program works.
    The program has trained about 75 Pacific bottlenose dolphins, with natural bio-sonar( 生物声呐) that track, better than any man-made device; and 35 California sea lions, with superb underwater eyesight.
    Not only do these trained marine mammals track and retrieve millions of dollars in U. S. military equipment, they are also helping to save lives.
    The Navy won’t disclose whether the dolphins and sea lions have effectively intercepted(拦截) terrorist【C6】________(attempt) to do harm to any U. S. facilities.
    "Either way, it serves as a deterrent(遏制的) effect," says Christian Harris, operations supervisor for the program. The mammals can be deployed via C-130 cargo aircraft to perform their missions anywhere in the world within 72 hours. They have been used in exercises from  Alaska to  Hawaii, 【C7】________( operate ) in great temperature and environmental ranges. They also have the capability to operate off vessels.
    Dolphins most recently were deployed in the Iraq war, 【C8】________( perform) mine detection and clearance operations in the Persian Gulf【C9】________(ensure) safe passage for humanitarian ships 【C10】________(deliver) aid. Some of these Iraq war "veterans" are now back home, tasked with a new mission: guarding nuclear submarines in their homeports of Bremerton, Washington, and Groton, Connecticut.
【C3】

选项

答案have recovered

解析 本题考查动词的时态。从原文可知,海狮已经恢复了价值成百上千万美元的鱼雷和仪器,故用现在完成时。
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