问题
选择题
Scientists say it still needs five or six years _____ it is possible for human patients to take this medicine. [ ]
A. since
B. before
C. after
D. when
答案
答案：B
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Scientists say it still needs five or six years _____ it is possible for human patients to take this medicine. [ ]
A. since
B. before
C. after
D. when
答案：B