单选题 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

【正确答案】 C
【答案解析】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。