A: The ball moves freely from A to O, and compresses the spring from O. According to Hooke's law, the elasticity of the spring gradually increases. According to Newton's second law, A = mg? Fm, the acceleration decreases, the direction is downward, and the ball accelerates with reduced acceleration. When the acceleration is zero, that is, when gravity and spring force are equal, the speed is the maximum; After that, the ball continued to move downward, and the elasticity was greater than gravity, slowing down, so A was correct;

B. From the analysis of A, it can be seen that the maximum speed of the ball is at point C (below point O) where gravity and elasticity are equal, and the ball falls freely from point A to point O V2=2ax: then v0 = 2g× 2x0 = 2gx0.

So VC > VO? Therefore, B is correct;

C, the maximum speed of the ball is equal to the elastic force under gravity. That is, mg = k △ X. So k = mg △ x and △ x < x0? So k > mgx0, so c is wrong;

D, due to the process from A to B, it can be obtained from the kinetic energy theorem: 0-0=mg(2x0+x0)-E bullet.

Solution: EP=3mgx0, so d is correct;

Wrong choice, so choose C.