The research they had been devoted to to be of great value to the cancer patients.A. is

问题补充：

The research they had been devoted to to be of great value to the cancer patients.

A. is proved

B. proves

C. has proved

D. proved

答案：

A. is proved

B. proves

C. has proved

(答案→)D. proved

解析：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。