Mary must keep her mind on her medical studies now so it would be years she could think

问题补充：

Mary must keep her mind on her medical studies now, so it would be years she could think about marriage.

A. before

B. when

C. since

D. until

答案：

(答案→)A. before

B. when

C. since

D. until

解析：考查连词。Before 在什么之前，才，就，when 当什么的时候，since 自从，until 直到，句意：现在玛丽把心思都放在医学研究上，因此，再过几年他才可能考虑结婚。故选A。