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

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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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