$A$ spherical balloon is expanding. If at any instant the rate of increase of its volume is $16$ times the rate of increase of its radius,then its radius at that instant is:

  • A
    $\frac{1}{\sqrt{\pi}}$
  • B
    $\frac{2}{\sqrt{\pi}}$
  • C
    $\frac{2}{\pi}$
  • D
    $\frac{4}{3\sqrt{\pi}}$

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