how many years will have to pass____scientists can succeed in providing a treatment for cancer?A whenB beforeC sinceD until为什么选B?而不选A,

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howmanyyearswillhavetopass____scientistscansucceedinprovidingatreatmentforcancer?AwhenBbeforeCsinceD

how many years will have to pass____scientists can succeed in providing a treatment for cancer?A whenB beforeC sinceD until为什么选B?而不选A,
how many years will have to pass____scientists can succeed in providing a treatment for cancer?
A when
B before
C since
D until
为什么选B?而不选A,

how many years will have to pass____scientists can succeed in providing a treatment for cancer?A whenB beforeC sinceD until为什么选B?而不选A,
在科学家找到治疗癌症的方法之前,还要过去多少年啊.
前面是一个时间段,而且没有确定性,A是指某个时刻,D比较偏重于确定

在科学家找到治疗癌症的方法之前,还要过去多少年啊。
前面是一个时间段,而且没有确定性,A是指某个时刻,D比较偏重于确定