一道英语语法选择题.Mary,turn off the water__you are brushing your teeth.

一道英语语法选择题.Mary,turn off the water__you are brushing your teeth.
---Mary,turn off the water__you are brushing your teeth.
---Sorry.I will do it at once.
A、until B、while C、though D、but
我知道B有道理.为什么不能选C啊~
选项出错了。为什么不能用during。