2 . In the past twelve months the research members ______ the effectiveness of many materials in a home building .
问题描述：
2 . In the past twelve months the research members ______ the effectiveness of many materials in a home building .
A . had studiedB . would studyC . studiedD . have studied
请说明一下，谢
答
D
因为是in the past twelve months
而且学习这个动作已经完成了
所以用过去完成时