Mechanical wave is the propagation of vibration in elastic medium (medium): the energy (vibration energy) from wave source is transmitted to each mass unit in turn from near to far along the propagation direction.

Different from the free vibration of a single particle, the vibration of each mass element in mechanical wave is forced vibration rather than free vibration, and the mechanical energy is not conserved, so the kinetic energy and potential energy (maximum shear deformation) of the mass element at the equilibrium position are the largest, and AB is wrong;

C only illustrates the propagation process of mechanical wave energy. When the mass element moves from the maximum displacement to the equilibrium position, the vibration energy increases, because the energy is obtained from the adjacent mass element, so C and D are not right.