A copper wire of length 36 m and diameter 2 mm is melted to form a sphere. The radius of the sphere (in cm) is :

Answer: Option B

Let the radius of the sphere be r cm
Then,
$$\begin{array}{rl}& \frac{4}{3}\pi r^3=\pi \times {\left(0.1\right)}^2\times 3600\\ & \iff r^3=36\times \frac{3}{4}\\ & \iff r^3=27\\ & \iff r=3cm\end{array}$$