A particle goes round a circular path with uniform velocity v. After describing half the circle, the average acceleration of the particle (if its radius is r) is

• A

  $\frac{{\mathrm{v}}^2}{\mathrm{r}}$

• B

  $\frac{{\mathrm{v}}^2}{\mathrm{\pi r}}$

• C

  $\frac{2{\mathrm{v}}^2}{\mathrm{r}}$

• D

  $\frac{2{\mathrm{v}}^2}{\mathrm{\pi r}}$