She asked me,"Do you want to send it by airmail or by ordinary mail?"改成间接引语 She said( )

题目

She asked me,"Do you want to send it by airmail or by ordinary mail?"改成间接引语 She said( )
答案是She asked me whether l wanted to send it by airmail or ordinary mail.
为什么把or去掉?

答案

呃...不是把or去掉,是把by去掉吧.whether.or.是表示选择句型,by其实是被省略掉了.