A:ab>0
B:ab >0
C:a<b
D:a>0>b
——What's your question?——I want to know _________.
Tom is ________ his pen, but he can't ________ it. Let's help him.
如图所示,在竖直向上的匀强磁场中,水平放置着一根长直流导线,电流方向指向读者,a、b、c、d是以直导线为圆心的同一圆周上的四点,在这四点中( )