【正确答案】Morphemes can be sub-classified according to their capacity of occurring alone．Those that may occur alone, that is, those which may make up words by themselves, are free morphemes． For example, dog, nation and close are free morphemes．In contrast, those that cannot occur alone and must appear with at least another morpheme, are called bound morphemes．For example, dis-, -ed are bound morphemes．

【答案解析】[解析] 本题考查自由语素和黏着语素的概念。根据能否单独出现，可以将语索分为两类。能单独出现也就是能单独成词的，是自由语素，与此相反，不能单独出现，必须跟至少一个其他语素共现的语素叫黏着语素。