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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