单选题Experiments in the photography of moving objects ______ in both the United States and Europe well before 1900.Awere conductingBhave been conductingChad been conductedDhad conducted
单选题
Experiments in the photography of moving objects ______ in both the United States and Europe well before 1900.
A
were conducting
B
have been conducting
C
had been conducted
D
had conducted
参考解析
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。
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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