单选题 By the time of 1969, there were many space explorations.
A. have been B. must be
C. had been D. must have been

【正确答案】 C

【答案解析】[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。