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：句意：科學(xué)家說可能過武或六年才有可能把這種藥在病人身上試驗。since 自從；  after 在......之后；before 在......之前；  when 當(dāng)......時候；It is /will be +時間段+before+（一般現(xiàn)在時的）句子“過多久才......”故選C。
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