Given the task definition and input, reply with output. In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: a hollow spherical metallic ball has an external diameter 6 cm and is 1 / 2 cm thick . the volume of metal used in the metal is : 
Options: ['a ) 47 1 / 5 cm 3', 'b ) 47 3 / 5 cm 3', 'c ) 47 7 / 5 cm 3', 'd ) 47 9 / 5 cm 3', 'e ) none of these']

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
b