The radius of a ring is R and its coefficient of linear expansion is $\alpha$. If the temperature of ring increases by $\theta$, then its circumference will increase by :

• A

  $\pi R\alpha \theta$

• B

  $2\pi R\alpha \theta$

• C

  $\pi R\alpha \frac{\theta }{2}$

• D

  $\pi R\alpha \frac{\theta }{4}$