Experiments in the photography of moving objects______in both the United States

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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

解析本题是对基本时态和语态的考查。实验是被实施的，所以本题中要用被动语态，其次实验发生在1990年之前，已经是过去完成的事情，因此要用过去完成时，结合选项可知，本题的答案为C。

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