Monomorphemic words are realized by single morphs such as_______

Monomorphemic words are realized by single morphs such as________ .
A、boss
B、cats
C、feet
D、trying
【正确答案】：A
【题目解析】：题干中提出单词素单词和它的例子，B和C中包含复数词素，D包含现在分词词素，只有A是单词素单词。