Assertion : Cobalt-60 is useful in cancer therapy. Reason : Cobalt- 60 is a source of $\gamma$-radiations capable of killing cancerous cells.

• Correct answer
  A

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

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

  If assertion is true, but reason is false
• D

  Both assertion and reason are false statements.

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