步进电动机工作时，每输入一个（），电动机就转过一个角度或前进一步。
参考答案：脉冲信号
Many people were invited to the party, but ____ them were absent for different reasons. [ ]
A. a great deal of
B. a number of
C. the number of
D. the amount of
栓剂的质量要求错误的是（）
A.药物与基质应混合均匀
B.只能起到局部的治疗作用
C.包装或贮藏时不变形
D.具有适宜的硬度
E.无刺激性
