题目内容

 The research they had been devoted to ______ to be of great value to the cancer patients.
A.is provedB.provesC.has provedD.proved
D
考查时态。句意:他们一直从事的研究证明对治疗癌症病人有巨大价值。they had been devoted to为后置定语从句。结合语境可知本句描述的是过去发生的动作,故用过去时态。选D。
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