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精英家教網 > 高中英語 > 題目詳情

Scientists say it may be five or six years _______ it is possible to test this medicine on human patients.

A. since B. after?

C. before D. when?

C?


解析:

此題考查幾個時間連詞在狀語從句中的區別。five or six years 是時間段,就應該找能表示點與點之間關系的連詞。since+過去時間點,主句常用現在完成時;after無法確定時間段;when只表點,不表段;所以用before表從現在到測試那一天。

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