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


  1. A.
    since
  2. B.
    after
  3. C.
    before
  4. D.
    when
C
解析:
句意為“科學家稱大概要過五到六年才有可能將這種藥用于人類臨床試驗”,表示“要過多久才……”用句型“it will be…before sb.do sth.”;表示“過了多久才……”用“it was…before sb.did sth.”。since只能用于“it is/has been…since sb.did sth.”的句型中,表示自從過去某個動作開始以來已經有多久了;when表示“在某個時刻”,它所修飾的主語的動作應該是一個時刻,而不是時間段。此題改為five or six years later就可以用when了。after表示在某動作之后,顯然不符合題意。
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