Two blocks `M_1` and `M_2` having equal mass are free to move on a horizontal frictionless surface. `M_2` is attached to a massless spring as shown in the figure. Initially `M_2` is at rest and `M_1` is moving towards `M_2` with speed v and collides head-on with `M_2`

Options

A. While spring is fully compressed all the KE of `M_1` is stored as PE of spring.
B. While spring is fully compressed the system momentum is not conserved, though final momentum is equal to initial momentum.
C. If spring is massless, the final state of the `M_1` is state of rest.
D. If the surface on which blocks are moving has friction, then collision cannot be elastic.