Experiments of this kind in both the U.S and Europe well before the Second World War.
A. had conducted
B. have conducted
C. have been conducted
D. had been conducted
网友回答
A. had conducted
B. have conducted
C. have been conducted
(答案→)D. had been conducted
解析：考查时态。根据句中时间状语before the Second World War可知用过去完成时，且“实验”是被“进行”，故选D。