定语从句的理解与能否改用which

This is the checkout where all the food is paid for.

1.请问已经是被动了，为何句子结尾还留一个for?

2.这个句子可以改用which吗？如此2个介词叠在一块吗？

This is the checkout which all food is paid for at?

This is the checkout where all the food is paid for.

你不能pay the food, 你只能pay for the food. 此时的pay是不及物动词，后接for才能再接food. 即pay for sth视为短语动词，被动语态则为 the food is paid for.

二个介词连续出现，很别扭，副词正好可以防止这样的情况。所以，本句不适合用which...at替换where。