The idiom diamond cut diamond is grammatically incorrect, for no

The idiom diamond cut diamond is grammatically incorrect, for normally the verb cut should take the third person singulars. However, we use it as it is. 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】：A