A spherical iron ball of radius \(10cm\) is coated with a layer of ice of uniform thickness that melts at a rate of \(50{cm}^{3}/min\). When the thickness of the ice is \(5cm\), then the rate at which the thickness (in cm/min) of the ice decreases is:
1
\(\cfrac { 1 }{ 9\pi }\)
2
\(\cfrac { 5 }{ 6\pi }\)
3
\(\cfrac { 1 }{ 18\pi }\)
4
\(\cfrac { 1 }{ 36\pi }\)