$A$ car is travelling with linear velocity $v$ on a circular road of radius $r$. If it is increasing its speed at the rate of $a \ m/s^2$,then the resultant acceleration will be

• A
  $\sqrt {\frac{v^4}{r^2} - a^2}$
• B
  $\sqrt {\frac{v^4}{r^2} + a^2}$
• C
  $\sqrt {\frac{v^2}{r^2} - a^2}$
• D
  $\sqrt {\frac{v^2}{r^2} + a^2}$