A, since the bottom of the bouncing ball just touches the ground, gravity is greater than elasticity, and people continue to accelerate downward, with mg-F=ma, f increasing and a decreasing. When the elastic force is equal to gravity, the acceleration drops to zero and the speed reaches the maximum. After that, due to inertia, people continue to fall, the elasticity of the bouncing ball is greater than gravity, people slow down, F-mg=ma, F gradually increases, and then A gradually increases. Therefore, in this process, people's speed first increases and then decreases, and acceleration first decreases and then increases, so the instantaneous power of gravity P=mgv, first increases and then decreases, and people's kinetic energy first increases and then decreases, so A is wrong, B is right and C is wrong;

D, the mechanical energy of the system composed of man and elastic ball is conserved, and the elastic potential energy of elastic ball is increasing, so the mechanical energy of man is decreasing, so D is correct;

So: BD.