I( )on the door for ten minutes now without an answer.

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问题 I( )on the door for ten minutes now without an answer.

选项 A、was knocking
B、am knocking
C、knocking
D、have been knocking

答案D

解析 本题考查动词时态的用法。for ten minutes now作时间状语,动词应用完成时态。
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