The idiom diamond cut diamond is grammatically incorrect, for normally the verb cut should take the third person singular -s. 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