According to the kinetic theory of gases,which of the following statements are correct?
$A$. Collisions are always elastic.
$B$. Heavier molecules transfer more momentum to the wall of the container.
$C$. Only a small number of molecules have very high velocity.
$D$. Between collisions,the molecules move in straight lines with constant velocities.

• A
  $A, C, D$
• B
  $A, B, C$
• C
  $B, C, D$
• D
  $A, B, D$