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

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

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

答案C

解析 根据句中的时间状语before 1990可知,实验是在1990年以前做的,所以本句应用过去完成时,表示动作发生在“过去的过去”。另外,根据句意可知,experiments和conduct为逻辑上的动宾关系,所以应用被动语态。故选C。
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