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 |
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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 |
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