本题考查词素变体。

Some morphemes are realized by more than one morph according to their position in a word. Such alternative morphs are known as allomorphs。有些词素根据它们在词中的位置不同可由一个以上的不同形素实现，这些不同形素叫词素变体。

B选项affixes“词缀”；C选项bound morphemes“粘附词素”；D选项prefixes“前缀法”。故本题选A。