mary lives in a small village.she has never traveled ____a train A on B by C at D through 53
问题描述：
mary lives in a small village.she has never traveled ____a train A on B by C at D through 53
答
选A 因为坐火车是在火车上的
如果省略掉这个a 就要选B by train 是固定搭配