Assertion $(A)$: If $\alpha=12^{\circ}, \beta=15^{\circ}, \gamma=18^{\circ}$,then $\tan 2 \alpha \tan 2 \beta+\tan 2 \beta \tan 2 \gamma+\tan 2 \gamma \tan 2 \alpha=1$.
Reason $(R)$: In $\triangle ABC$,$\tan \frac{A}{2} \tan \frac{B}{2}+\tan \frac{B}{2} \tan \frac{C}{2}+\tan \frac{C}{2} \tan \frac{A}{2}=1$.
Which of the following is true?

• A
  Both $(A)$ and $(R)$ are true and $(R)$ is the correct explanation of $(A)$
• B
  Both $(A)$ and $(R)$ are true,but $(R)$ is not the correct explanation of $(A)$
• C
  $(A)$ is true,but $(R)$ is false
• D
  $(A)$ is false,but $(R)$ is true