Experiments of this kind in both the U.S. and Europe well before the Second World War started.
A. have conducted
B. have been conducted
C. had conducted
D. had been conducted
网友回答
A. have conducted
B. have been conducted
C. had conducted
(答案→)D. had been conducted
解析：句意：在第二次世界大战之前这种实验就在美国和欧洲做过。因为时间状语before the Second World War started.这里的谓语动词是过去式，conduct是在这个动作之前的发生的动作，故用过去完成时，又因为experiments与conduct之间是被动关系，故选D。