题干
It is important ______ students ______ carefully in class .
A：for, listening.
B：of , listen
C：for, to listen
D：of , to listen.
答案(点此获取答案解析）
C
It is important ______ students ______ carefully in class .
A：for, listening.
B：of , listen
C：for, to listen
D：of , to listen.
表示电流垂直向外流出，
表示电流垂直向里流进．接着把磁体的 N、S极对调，记录的现象如图2乙所示．
