问题
选择题
Paul doesn't have to be made _____. He always works hard. [ ]
A. learn
B. to learn
C. learned
D. learning
答案
答案：B
搜题请搜索小程序“爱下问搜题”
Paul doesn't have to be made _____. He always works hard. [ ]
A. learn
B. to learn
C. learned
D. learning
答案：B
L。一个质量为m、电荷量为+q的带电粒子（不计重力）以初速度v0，从P处垂直对角线BD进入磁场，并从DC边上的Q点垂直于DC离开磁场，试求：