Scientists say it may be five or six years it is possible to test this medicine to human

问题补充：

Scientists say it may be five or six years it is possible to test this medicine to human patients.

A. since

B. after

C. before

D. when

答案：

A. since

B. after

(答案→)C. before

D. when

解析：考查固定句型：It will be/may be+一段时间+before表示要过多长时间才做某事，由此题看是填before,意思是:科学家说要过五年或十年才能在人身上试验这药.选C。