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 ofidioms 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】：在很大程度上习语的结构是不能改变的，in a brown study是陷入深思的意思，其中的颜色词brown是不可以换成其他颜色的。因此，C为答案。
