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(gè)時(shí)間連詞在狀語從句中的區(qū)別。five or six years 是時(shí)間段，就應(yīng)該找能表示點(diǎn)與點(diǎn)之間關(guān)系的連詞。since+過去時(shí)間點(diǎn)，主句常用現(xiàn)在完成時(shí)；after無法確定時(shí)間段；when只表點(diǎn)，不表段；所以用before表從現(xiàn)在到測試那一天。
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