friend and enemy

We must learn $\frac{\text{to}}{\text{A}}$ distinguish between $\frac{\text{between}}{\text{B}}$ , $\frac{\text{a}}{\text{C}}$ friend and $\frac{\text{enemy}}{\text{D}}$ . 这道试题判 C 为错误(应去掉不定冠词 a)。但是, friend 和 enemy 都是可数名词, 它们是应当使用冠词的呀!