Unlike free phrases, the structure of an idiom is to a large ext

Unlike free phrases, the structure of an idiom is to a large extent_________ , as is shown in the idiom “in a brown study”, which cannot be changed into “in a brown hall”.
【正确答案】：unchangeable
【题目解析】：习语和自由词组做了对比，习语结构是不变的，而自由词组则可以有变换。