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

问题补充：

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：

A. since

B. after

(答案→)C. before

D. when

解析：考查连词。Since自从，after后来，before在什么之前，才，就， when当什么时候，句意：科学家们说可能五年或六年之后才有可能在人类病人上检测这种药物。根据语境选C。