—Excuse me, madam. Could you please tell me _________ the bus arrives? —Sure. At 9:00.
A:where
B:when
C:why
D:how
某电解池中,若在2s内有1.0×1019个二价正离子和2.0×1019个一价负离子通过某截面,那么通过这个截面的电流是( )