单选题
Experiments in the photography of moving objects______in both the united states and Europe before 1990.
A、
have been conducting
B、
were conducting
C、
had been conducted
D、
are conducted
【正确答案】
C
【答案解析】
[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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