Experiments of this kind ___in both the U.S and Europe well before the Second World War.
A．had conductedB．have conductedC．have been conductedD．had been conducted
D
考查時態(tài)。根據(jù)句中時間狀語before the Second World War可知用過去完成時，且“實驗”是被“進行”，故選D。
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