If at any instant $t$, for a sphere, $r$ denotes the radius, $S$ denotes the surface area and $V$ denotes the volume, then what is $\frac{d V}{d t}$ equal to?

• A

  $\frac{1}{2} S \frac{d r}{d t}$
• Correct answer
  B

  $\frac{1}{2} r \cdot \frac{d S}{d t}$
• C

  $r \frac{d S}{d t}$
• D

  $\frac{1}{2} r^2 \frac{d S}{d t}$

Hence, the correct answer is option: 'B'.