As the chief function of functional words is to express the relation between notions, the relation
between words as well as sentences, they are known as ( ) words.
【正确答案】：FORM
【题目解析】：本题考查虚词的含义。功能词本身不具有实义，也称为虚词。虚词就是表示两个实义词之间以及词与词，句子与句子之间的关系，也可称为形式词。比如：介词、连词、助动词、冠词。form：形式。
