问题
选择题
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
答案
答案：C
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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
答案：C
缺失，欲采用粘结桥修复，关于基牙的预备以下不正确的是（）