简要说明指令性语句与指示性语句的区别。

A. 电压及对称性
B. 频率
C. 波形
D. 发电机阻抗
E. 电动机Y-△运行

A. should be coiled in a clockwise direction
B. should be coiled in a counterclockwise direction
C. may be coiled either clockwise or counterclockwise
D. should never be coiled