When in a brown study is used as an idiom, brown cannot be repla

When in a brown study is used as an idiom, brown cannot be replaced by red, green, etc. This shows that（ ）
A、many idioms are grammatically unanalysable
B、the word order of idioms cannot be inverted or changed
C、the structure of an idiom is to a large extent unchangeable
D、the constituents of an idiom cannot be deleted or added to
【正确答案】：C
【题目解析】：本题主要考查习语结构的稳定性。当in a brown study（沉思）被用作成语时，brown（棕色）不能被red（红色）、green（绿色）等所取代。答案为C。