Experiments of this kind____in both the U.S.and Europe well before the World War.
A.have conducted B.have been conducted
C.had conducted D.had been conducted
句子大意：在世界大战之前这种实验已经在美国和欧洲两地进行的很好了.
通过时间状语“before the World War”可判断是过去的时间,所以可排除AB!
再有就是Experiments与conduct之间是被动关系,所以选D是对的!