Assertion: $\mathrm{ZnO}$ becomes yellow when it is heated. Reason : Metal excess defect arises due to excess of $\mathrm{Zn}^{2+}$ ions which move to interstitial sites and the electron moves to neighbouring interstitial sites.

• 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'.