23+(-17)+(+7)+(-13)=______.
23+(-17)+(+7)+(-13),
=[(-17)+(-13)]+(23+7),
=-30+30,
=0,
故答案为0.
He _____ late for school. [ ]
A. is often
B. often is
C. does often
D. often does
在特别潮湿及严重污秽等恶劣环境下,隧道内接触网带电体距接地体的静态间隙困难值为()。
A、240mm
B、160mm
C、300mm
D、200mm