A. If the magnetism of the magnet is strong enough, the movement of the magnet in the copper tube will be subject to greater resistance. The reason is that when the magnet moves, the magnetic flux of the steel pipe will change, and the induced magnetic field will hinder the change of the original magnetic field, so the movement time will be longer, but it will definitely decrease, so A is wrong and B is right;

C, in the process of the magnet moving in the copper tube, although the air resistance is not considered, in this process, the ampere force does work to generate heat energy, so the mechanical energy is not conserved, so C is wrong;

D, if the copper tube is replaced by a plastic tube, the magnet will not be resisted by Ampere force, so the time of coming out is unchanged, so D is wrong;

So choose: B.