Huizhou Medical Equipment Compression Spring Quotation

A, because the object is acted by elastic force, elastic force does work on the object, so the mechanical energy of the object is not conserved, so A is wrong;

B, the system composed of objects and springs only uses gravity to do work, so the mechanical energy of the system is conserved, so B is correct;

C, the mechanical energy of the system is conserved, so when the object compresses the spring to the shortest, the mechanical energy remains unchanged, so C is wrong;

D, the object first makes a free-falling motion, and the acceleration decreases but the speed increases after contacting with the spring; When gravity and spring force are equal, the speed of the object reaches the maximum, so d is correct;

So choose BD.