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!