For the words collection, idealistic, ex-prisoner, only collect,

For the words collection, idealistic, ex-prisoner, only collect, ideal, prison are _____.
A、bound morphemes
B、free morphemes
C、grammatical words
D、functional words
【正确答案】：B
【题目解析】：morpheme 可以分为自由词素和黏着词素，自由词素也可以叫作自由词根，是可以独立成词的词素。functional words 指的是虚词。