单选题 The new e-ticketing system, ______ was installed last month, is working well.
A. what
B. whom
C. where
D. which

【正确答案】 D
【答案解析】四个选项中只有关系代词which可以指代事物(system)。选项(A)what不是关系代词，选项(B)whom指人，而选项(C)where指地点。