During an elastic collision between two bodies,which of the following statements are correct?
$I$. The initial kinetic energy is equal to the final kinetic energy of the system.
$II$. The linear momentum is conserved.
$III$. The kinetic energy during $\Delta t$ (the collision time) is not conserved.

• A
  $II$ and $III$ only
• B
  $I$ and $III$ only
• C
  $I, II$ and $III$
• D
  $I$ and $II$ only