How long do you think it will be scientists succeed in providing a cure for cancer?
A. when
B. after
C. since
D. before
网友回答
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。