Idioms nominal in nature have a(n)（）as the key word in each and

Idioms nominal in nature have a(n)（）as the key word in each and function as a noun in sentences.
A、verb
B、adjective
C、preposition
D、noun
【正确答案】：D
【题目解析】：这一类习语中，每一个习语中都会有一个关键词，并且有着名词的功能 eg. White elephant