A, cd moves to the right, ab does not move, the magnetic flux through the loop increases, and the original magnetic field direction is inward. According to Lenz's law, the current through ab is from A to B, so A is correct.

B, C and cd cut the magnetic induction wire to generate induced electromotive force, which is equivalent to the power supply, and ab is the external circuit. Because the resistance of the guide rail is ignored, the voltage across ab is equal to the voltage across cd. That is Uab=Ucd .. So, B is right and C is wrong.

The voltage across D and cd is external voltage, which is less than electromotive force B and greater than V, so D is wrong.

Therefore: AB