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

解析本题考查时态。句意：在1990年之前，在美国和欧洲就已经进行了移动物体摄影的实验。根据题干中的时间“before 1990(1990年之前)”判断该句时态为过去完成时。选项A为现在完成进行时，选项B为过去进行时，选项D为现在时，所以本题选C。

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Experimentsinthephotographyofmovingobjects______inboththeUnitedState