He realized she was crying what he had sad.A. becauseB. because ofC. asD. since

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A. because
(答案→)B. because of
C. as
D. since
解析：考查名词性从句。what he had sad是名词性从句，而连词because, as和since
不能引导名词性从句，所以选B，because of ，介词of后面引导的是宾语从句。