1.C
定语从句中,关系代词which代替先行词plane,在定语从句中充当介词by的宾语.
即：by which he came back
= by plane he came back
= he came back by plane
1.I ask him which is the way to the bus stop.
2.The policeman asked the madam if/whether there was something wrong,
3.She asked who that boy was.
4.The student said that he/she hadn't brought his/her dictionary.第一题为什么不选that?定语从句中的关系代词that不能放在介词（by）之后，所以不能用that.确定间接引语的答案吗？第2题还可以写成：The policeman asked if/whether there was something wrong with the madam/lady.