Assertion : Mass is not conserved, but mass and energy are conserved as a single entity is called mass-energy. Reason: Mass and energy are inter-convertible in accordance with Einstein's relation, $E=m c^2$.

• Correct answer
  A

  If both assertion and reason are true and reason is the correct explanation of assertion.
• B

  If both assertion and reason are true but reason is not the correct explanation of assertion.
• C

  If assertion is true but reason is false.
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'A'.