Find the Dimensions of a Rectangular Box of Maximum Volume $84$

Multiple Choice

Find the dimensions of a rectangular box of maximum volume such that the sum of the lengths of its 12 edges is $84$

A) $7$ , $84$ , $7$
B) 4, 8, 16
C) $7$ , $7$ , $7$
D) 32, $7$ , 16
E) 32, 32, 32