题目类型：
单选题
题目内容
By notion, words can be grouped into content words and functional words. Which of the following words belongs to functional words?
正确答案
D
题目解析
本题考查实词虚词的内容，实词本身是有实际意思的，虚词本身没有实在意思，如and，on, but, do, be 等介词、连词等，故选D。