Experiments in the photography of moving objects ______ in both the United State

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问题 Experiments in the photography of moving objects ______ in both the United States and Europe well before 1990.

选项 A、have been conducting
B、were conducting
C、had been conducted
D、are conducted

答案 C

解析该题测试考生的基本时态和语态概念。首先，这道题必须是被动语态。其次，试验是在1990年之前做的，所以必须用过去完成时态。

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