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 |
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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 |
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