Scientists say it may be one year or so it is possible to test the new medicine on human

问题补充：

Scientists say it may be one year or so it is possible to test the new medicine on human patients.

A. when

B. after

C. before

D. since

答案：

A. when

B. after

(答案→)C. before

D. since

解析：词义辨析。句意：科学家们说在有可能把这种新药用于人类前可能需要大约一年的时间。所以选C。