It takes at least five years to ten years_____ it is possible to test this medicine on human patients. 　　<br />A.before<br>B.since<br>C.after<br>D.when

It takes at least five years to ten years_____ it is possible to test this medicine on human patients. 　　
A.before
B.since
C.after
D.when

出自：国家开放大学英语语法

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