(20 14? Shaoguan no.1 model) a light spring, fixed at point o on the ceiling, with the original length of l, as shown in the figure, an object with mass of m is vertical from point a.

A. In the process from A to C, for the system composed of mass and spring, only gravity and elastic force of spring do work, and the mechanical energy of the system is conserved, that is, the sum of gravitational potential energy, kinetic energy and elastic potential energy of mass remains unchanged, while elastic potential energy of spring increases, so the sum of gravitational potential energy and kinetic energy decreases, so A is wrong.

B, in the process from B to C, the mechanical energy of the system is conserved, that is, the sum of gravitational potential energy and kinetic energy of the object and the elastic potential energy of the spring remain unchanged, while the gravitational potential energy of the object increases and the sum of elastic potential energy and kinetic energy decreases, so B is correct;

C. In the process from A to C, for the object M, due to the elasticity of the spring, its mechanical energy is not conserved, so C is wrong.

D in the process from b to c, for the system composed of mass and spring, only gravity and elastic force of spring do work, and the mechanical energy of the system is conserved. Therefore, d is correct.

So: BD.