When the wireframe moves to the position shown in the figure, it moves in a straight line at a uniform speed, that is, the force is balanced at this time.

That is mg=F amp =B2L2VR.

When the wireframe falls to the position shown in the figure:

At this time, the magnetic flux in the wire frame is constant, there is no induced electromotive force and current, and it is not affected by ampere force, but only by gravity, so the speed of the object will inevitably increase when it moves in a straight line with uniform acceleration. So the upper frame comes out of the magnetic field faster than the lower frame enters the magnetic field, and it takes less time than T, so A is wrong and B is right.

Because the wireframe moves in a straight line at a uniform speed when it enters the magnetic field, the stress is mg=F-A =B2L2VR.

So gravity does as much work as overcoming Ampere's force, that is, the work done by gravity is equal to the heat generated.

The upper frame moves faster in the magnetic field than when it first entered the frame, and the electromotive force and current generated are greater than the initial value, so the upward ampere force is greater than gravity, so the heat generated is greater than mgh, so C is correct and D is wrong.

Therefore: BC