高等继续教育 / 专科英语II
正确率：100%
题型描述: 单选题
Scientists say it may be five or six years ＿＿＿＿＿＿＿＿ it is possible to test this medicine on humans.
A.before
B.when
C.since
D.after
Scientists say it may be five or six years ＿＿＿＿＿＿＿＿ it is possible to test this medicine on humans.
A.before
B.when
C.since
D.after
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