When______his homework,he heard someone singing.

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问题 When______his homework,he heard someone singing.

选项 A、he does
B、he doing
C、doing
D、did

答案C

解析 由when等引导的状语从句,如果从句主语与主句的主语一致或是it,从句谓语动词又含be动词时,主语和be动词常常被省略,就变成“when+v-ing/v-ed/adj. /介词词组”的结构。
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