$Assertion$ : In an elastic collision of two billiard balls,the total kinetic energy is conserved during the short time of oscillation of the balls (i.e.,when they are in contact).
$Reason$ : Energy spent against friction does not follow the law of conservation of energy.

• 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.