问一道英语语法题~

题目

问一道英语语法题~
China's new food law provides for a food recall (召回) system ______ producers have to stop production if their food isn't up to standards.
A.where B.that C.when D.which
请说明原因~
如果选that 的话，为什么where 不对？

答案

选B,这是个同位语从句,后面producers.一直到结束是对food recall system 的说明 同位语从句只能用that引导,这里不是定语从句,注意 这是同位语从句啊,从句里 producers have to stop production if their food isn't ...