$A$ particle is moving along the circle $x^2 + y^2 = a^2$ in an anticlockwise direction. The $x-y$ plane is a rough horizontal stationary surface. At the point $(a \cos \theta, a \sin \theta)$,the unit vector in the direction of friction on the particle is:

• A
  $\cos \theta \hat{i} + \sin \theta \hat{j}$
• B
  $-\left( \cos \theta \hat{i} + \sin \theta \hat{j} \right)$
• C
  $\sin \theta \hat{i} - \cos \theta \hat{j}$
• D
  $\cos \theta \hat{i} - \sin \theta \hat{j}$