If $\left(\frac{\sin 3 \theta}{\sin \theta}\right)^2-\left(\frac{\cos 3 \theta}{\cos \theta}\right)^2=a \cos b \theta$, then $a: b=$

• Correct answer
  A

  $4: 1$
• B

  $8: 1$
• C

  $3: 2$
• D

  $2: 1$

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