Scientists say it may be five or six years _______ it is possible to test this medicine on human patients.
A．sinceB．a(chǎn)fter?
C．beforeD．when?
C?
此題考查幾個時間連詞在狀語從句中的區(qū)別。five or six years 是時間段，就應(yīng)該找能表示點與點之間關(guān)系的連詞。since+過去時間點，主句常用現(xiàn)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(xiàn)在到測試那一天。
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