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 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
【正确答案】：A
【题目解析】：虽然这句习语从语法上分析是不正确的，但我们依然这样在用，说明了习语的一个特点就是不讲究语法。