$Assertion$: $n$ small balls each of mass $m$ collide elastically each second on a surface with velocity $u$. The force experienced by the surface is $2mnu$.
$Reason$: On elastic collision,the ball rebounds with the same velocity.

• A
  If both $Assertion$ and $Reason$ are correct and the $Reason$ is a correct explanation of the $Assertion$.
• B
  If both $Assertion$ and $Reason$ are correct but $Reason$ is not a correct explanation of the $Assertion$.
• C
  If the $Assertion$ is correct but $Reason$ is incorrect.
• D
  If both the $Assertion$ and $Reason$ are incorrect.