(20 12? In the figure, MN and GH are long enough smooth parallel metal rails, and the metal bars AB and CD are vertically placed on the two rails, so the whole device is in the same position.
When the CD rod moves to the right, induced electromotive force and induced current are generated in the whole loop, and then the CD rod is decelerated to the left by ampere force, and the AB rod is accelerated by ampere force. When the speed of the two rods is equal, the induced current in the whole loop is zero, and the two rods move linearly at the same speed. So B is right and A, C and D are wrong.
So choose B.