问题
选择题
Scientists say it may be five or ten years________ it is possible to test this medicine on human patients.[ ]
A. since
B. when
C. after
D. before
答案
答案：D
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Scientists say it may be five or ten years________ it is possible to test this medicine on human patients.[ ]
A. since
B. when
C. after
D. before
答案：D