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 |
推荐套卷
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 |