$A$ particle of mass $m$ is moving in a circular path of constant radius $r$ such that its centripetal acceleration $a_c$ is varying with time $t$ as $a_c = k^2 r t^2$. The power delivered to the particle by the forces acting on it is:

• A
  $2\pi m k^2 r^2 t$
• B
  $m k^2 r^2 t$
• C
  $\frac{m k^4 r^2 t^5}{3}$
• D
  Zero