Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.
A．sinceB．a(chǎn)fterC．beforeD．when
C

試題分析：考查固定句型：It will/may be+一段時間+before…,要過多長時間才/就…,句意：科學(xué)家說要再過五六年這種藥物才能在人身上試驗。故選C。
點評：It won’t be long before….，要不了多久就會…和這個句型相似的是：It is+一段時間+since…，這兩個句型的時態(tài)是不同的，要看清楚。
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