The base of a solid is formed by the region ( y = 1/4x² ) and ( y = 4 ). Cross sections perpendicular to the y-axis are semi-circles. Find the volume of the solid that is formed.

a) ( 256π/3 );
b) ( 512π/3 );
c) ( 1024π/3 );
d) ( 2048π/3 )