If the magnetic field is uniform, according to the law of conservation of energy, we can get

The lost mechanical energy △ e = MGB+12mv2-MGA = mg (b-a)+12mv2.

If the magnetic field is non-uniform, the metal ring will eventually stop at the lowest point, which is obtained according to the law of conservation of energy.

Lost mechanical energy △E=mgb+ 12mv2.

Therefore: BC