Scientists say it may be five or six yea
题型：选择题
问题：
Scientists say it may be five or six years ________ it is possible to test this medicine on human patients. [ ]
A. since
B. after
C. before
D. when
Scientists say it may be five or six years ________ it is possible to test this medicine on human patients. [ ]
A. since
B. after
C. before
D. when