They demanded that the books________ to the school library at once
A.return
B.should return
C.be returned
D.would be returned
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C解析试题分析：考查虚拟语气。表示命令、建议、要求等的词demand, mand, order, request, require, suggest(建议)，insist(坚持要求)等，后面的宾语从句用虚拟语气，即，should+动词原形。should可省略。根据demanded可知，是过去时，排除A；根据（should）+do排除D；books 和return是动宾关系，用被动语态，排除B。故选C。考点：考查虚拟语气