单选题,9分] Scientists say it may be five or ten years _______ it is possible to test this medicine on human patients. A.since B.before C.after D.when

单选题,9分] Scientists say it may be five or ten years _______ it is possible to test this medicine on human patients.
A.since
B.before
C.after
D.when

出自：河南理工大学—大学英语3

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