If $\mathbf{P}+\mathbf{Q}=\mathbf{R}$ and $\left|\mathbf{P}\right|=\left|\mathbf{Q}\right|=\sqrt{3}$ and $\left|\mathbf{R}\right|=3$, then the angle betweenP and Q is

• A

  $\pi /4$

• B

  $\pi .6$

• C

  $\pi /3$

• D

  $\pi /2$