摘要：3．While doing my homework. . A．I heard the doorbell ring B．the doorbell rang C．my mother came in D．my mother was cooking 答案与解析:A 只有当状语从句的主语与主句主语一致时.才能省略从句中的主语和be动词.所以主句主语必须是I.因此选A项.

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