they have reached an agreement____that will benefit both sides

题目

they have reached an agreement____that will benefit both sides
Athat Bwhat Cone D it
请说下理由,

答案

我的答案是C,空格处可以用which来找出对应可以取代的词,这样A和B就可以排除,剩下C和D,
which=one that or it that,从语法习惯上,我选择了C,one that,代替which,指代前半句的事.