Gas is being pumped into a spherical balloon at the rate of $30 \ ft^3 / min$. The rate at which the radius increases when it reaches the value $15 \ ft$,is:

  • A
    $\frac{1}{30 \pi} \ ft / min$
  • B
    $\frac{1}{15 \pi} \ ft / min$
  • C
    $\frac{1}{20} \ ft / min$
  • D
    $\frac{1}{25} \ ft / min$

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