今天做了篇完形填空，有一个知识点搞不懂，望各位大虾相助，如下

In some cities school buses are used only when children live more than a mile from school.When children are too young to walk (  ) far ,their mothers take turns to drive them to school.

A. any       B.that        C.the      D.much

答案：Ｂ

ＷＨＹ？？？？？

我感觉好像不应该是B,我也想不通。

我感觉是选b

that代指a mile，不过具体说不出为什么

Thank you all the same.