The days _____ you could travel without a passport are a thing o

The days _____ you could travel without a passport are a thing of the past.
A、in which
B、on which
C、of which
D、at which
【正确答案】：A
【题目解析】：选项中的which引导后边的you could travel without a passport修饰the days，同时，the days表示是一段时间所以which前加介词in构成从句的状语。