B, the spring's elastic potential energy with the spring's deformation variable increase and increase, from the figure know that the spring is shortened first and then restored to the original length and finally elongation, so the spring's elastic potential energy first increase and then decrease after the increase only right, so B error;
C, the whole process of
C, the whole process of the ring's gravitational potential energy decreases mgh, according to the conservation of mechanical energy of the system, the elastic potential energy increases mgh, so the spring elasticity of the work done by -mgh, so C is correct;
D, the whole system of mechanical energy conservation, that is, the elastic potential energy of the spring, the kinetic energy of the ring and the sum of the potential energy of gravity to keep the same, so the spring's potential energy of elasticity is the maximum, the minimum of the ring's kinetic energy and the sum of the gravitational potential energy, so D Correct;
The choice: CD.