问题
选择题
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
答案：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。