B, for the A object, B object and the spring composed of the system, only the spring elastic force and gravity to do the work, the system's mechanical energy conservation, due to the spring's potential energy of elasticity is increasing, so the A object with the system composed of the B object of mechanical energy continues to decrease, so B error;
C, A object, B object and the spring of the system composed of mechanical energy conservation, so the reduction of mechanical energy of the B object is equal to the increase in mechanical energy of A and the increase in elastic potential energy of the sum, so the reduction of mechanical energy of the B object is greater than the increase in elastic potential energy of the spring, so C error.
D, due to the spring's tension is less than the tension of the wire, after greater than the tension of the wire, A first accelerated after the deceleration, when the spring's tension and the size of the tension of the wire is equal to the speed of the A maximum, kinetic energy is the maximum, at this time, the acceleration of A is zero, B's acceleration is zero, the tension of the wire is equal to the force of gravity of the B, it can be seen that the spring's tension is equal to the gravitational force of the B object, the A object of the kinetic energy is maximum. Therefore, D is correct.
The choice: D
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