A:4.6m
B:4.8m
C:5m
D:5.2m
—Look,here comes your dream girl. Invite her to dance.
—________ What if she refuses me?
奥斯特实验说明了____;通电螺线管附近小磁针如图所示,则电源的a端是____极(选填“正”或“负”).