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
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(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。