Complete the following statements with proper words or expressions according to the course book.Based on the criterion of ( ) functions, idioms may be classified into five groups.
【正确答案】：GRAMMATICAL
【题目解析】：根据语法功能的标准，习语可分为五大类：名词性习语 (idioms nominal in nature)、形容词性习语 (idioms adjectival in nature)、动词性习语 (idioms verbal in nature)、副词性习语 ( idioms adverbial in nature)和句式习语 (sentence idioms)。
