When in a brown study is used as an idiom， brown cannot be replaced by red， green， etc. This shows that（） .

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