In the process of small L: judging from Lenz's law, the direction of induced current in counterclockwise direction is positive; The magnitude of induced electromotive force is E 1=BLv, and the magnitude of induced current is I1= e1r = blvr;

In the process of L-rL: judging from Lenz's law, the direction of induced current in counterclockwise direction is positive; The magnitude of induced electromotive force is Er=rBLv, and the magnitude of induced current is IR = err = rblvr.

In the process of rL-3L, judging from Lenz's law, the direction of induced current is clockwise negative; The magnitude of induced electromotive force is E3=3BLv, and the magnitude of induced current is i3 =-e3r =? 3 blvr。 Therefore, B is correct.

So choose B.