$A$ particle moves along an arc of a circle of radius $R$. Its velocity depends on the distance covered as $v = a\sqrt{s}$,where $a$ is a constant. Then the angle $\alpha$ between the vector of the total acceleration and the vector of velocity as a function of $s$ will be:

• A
  $\tan \alpha = \frac{R}{2s}$
• B
  $\tan \alpha = \frac{2s}{R}$
• C
  $\tan \alpha = \frac{2R}{s}$
• D
  $\tan \alpha = \frac{s}{2R}$