i want to complain the poor service of to in 理由,

题目

i want to complain the poor service of to in 理由,
i want to complain the poor service。
　　　　　　　　of　　　to　　　in　　　理由，

答案

答案是 of
complain of sth = 对某事投诉
是固定搭配.
通常也说 complain about sth.
我是加拿大人,前英语老师.