NASA's Mars detectot,Opportunity,succeeded in finding signs that water once existed on the planet．It is now believed that the planet ________ life．


  1. A.
    should once support
  2. B.
    should once have supported
  3. C.
    could once support
  4. D.
    could once have supported
B
解析：
考查情態(tài)動詞的用法，根據(jù)前句信息，那個星球上曾經(jīng)有水存在的跡象已經(jīng)被成功找到了，后句的語氣應(yīng)該很肯定的，談?wù)摰氖沁^去的事情，選的情態(tài)動詞后要出現(xiàn)完成形式，所以在B和D里選，根據(jù)語氣判斷是比較肯定的，所以選擇B，意思是，現(xiàn)在人們相信那星球上應(yīng)該是存在過生命的。
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