问题
单项选择题
Experiments in the photography of moving objects () in both the United States and Europe well before 1990.
A. have been conducting
B. were conducting
C. had been conducted
D. are conducted
答案
参考答案：C
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。