Experiments of this kind ________ in both the U. S. and Europe well before the Second World War.[ ]
A. have conducted
B. have been conducted
C. had conducted
D. had been conducted
答案:D
Experiments of this kind ________ in both the U. S. and Europe well before the Second World War.[ ]
A. have conducted
B. have been conducted
C. had conducted
D. had been conducted
答案:D