题目

Only when class began _______that he had left his book at home. 　 A. will he realize 　 B. he did realize 　 C. did he realize 　 D. should he realize 答案： C 解析:  "only+状语"放于句首，应构成倒装句，主语前要加助动词，由时态看为一般过去时。

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