Problem: if the space diagonal of cube c is 5 inches long , what is the length , in inches , of the diagonal of the base of cube c ? 
Options: ['a ) 5 / ( √ 6 )', 'b ) 5 · √ ( 2 / 3 )', 'c ) 5 · √ ( 3 / 2 )', 'd ) 5 · √ 3', 'e ) 5 · √ 6']
b