The material of a sphere of radius r is melted and recast into a hollow cylindrical sell of thickness a and outer radius b. What is its length asssuming that no material is lost in recasting?

A

$$\frac{4r^{3}}{3a(2a-b)}$$

B

$$\frac{4r^{3}}{3a(b+a)(b-a)}$$

C

$$\frac{4r^{3}}{3a(b-a)}$$

D

$$\frac{4r^{3}}{3a(2b-a)}$$