"Where is your mother going ___?"横线上怎么能添"to"呢?

题目

"Where is your mother going ___?"横线上怎么能添"to"呢?
这个问题出现在新概念3第16页的练习题中,答案给了"to".怎么回事呢?

答案

Where is your mother going to?
=Where is your mother going ?
前句不常用,一般用于书面语,但合乎语法
PS:我们新概念的老师有提过哦