题干
．QQ, you can chat with your friends, makes communication easy.
A：why
B：which
C：that
D：where
答案(点此获取答案解析）
D
动势大小为E。忽略涡流损耗和边缘效应。关于
、
的大小和铜棒离开磁场前两端的极性，下列判断正确的是