The train,I was travellling _ was late.选A为什么错

题目

The train,I was travellling _ was late.选A为什么错
A.where B.in which c.on which D.that

答案

正确答案是：C.on which 因为这个是定语从句中“介词+ which” 的形式
原句：I was travelling on the train.“on the train”表示坐火车