By the time of 1969, there were many space explorations.A、have beenB、must beC、

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问题 By the time of 1969, there were many space explorations.

选项 A、have been
B、must be
C、had been
D、must have been

答案 C

解析截至1969年。已有大量太空探索活动。本题考查时态的用法。1969为一个过去的时间点，By the time of 1969“截至1969”则表示过去的过去，因此要用过去完成时，选C项。

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Bythetimeof1969,thereweremanyspaceexplorations.A、havebeenB、mustbeC、