问题补充：
They demanded that the books to the school library at once.
A. return
B. should return
C. be returned
D. would be returned
答案：
A. return
B. should return
(答案→)C. be returned
D. would be returned
解析：考查虚拟语气。表示命令、建议、要求等的词demand, command, 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。
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