问题
选择题
The computer is very useful in our life, ________it? . [ ]
A. isn't
B. wasn't
C. hasn't
D. doesn't
答案
答案：A
搜题请搜索小程序“爱下问搜题”
The computer is very useful in our life, ________it? . [ ]
A. isn't
B. wasn't
C. hasn't
D. doesn't
答案：A
Q，q对原电场无影响)可看做点电荷的小球，从B点无初速释放，如果小球质量为m，下滑过程中带电荷量不变，求：