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
D
考查時(shí)態(tài)。根據(jù)時(shí)間狀語in the past twelve months在過去的12個(gè)月，用現(xiàn)在完成時(shí)，故選D。
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