How long do you think it will be scientists succeed in providing a cure for cancer?A. whe

问题补充：

How long do you think it will be scientists succeed in providing a cure for cancer?

A. when

B. after

C. since

D. before

答案：

A. when

B. after

C. since

(答案→)D. before

解析：句意:你认为还需多长时间科学家就能成功提供治癌症的方法？when当……时候；after在……之后；since自从；it will be some time before...再过多长时间就。根据句意故选D。