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

解析时态应用根据时间状语by the time of 1969可判断本句表示过去某时刻前发生的动作或状态，应用过去完成时；因此答案为C。

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