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 1900.

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

答案 C

解析由后面的时间可知应填过去时态，而“before+时间”通常与完成时连用。

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