Problem: a closed cylindrical tank contains 36 pie cubic feet of water and its filled to half its capacity . when the tank is placed upright on its circular base on level ground , the height of water in the tank is 4 feet . when the tank is placed on its side on level ground , what is the height , in feet , of the surface of the water above the ground ? 
Options: ['a ) 0 feet', 'b ) 1 feet', 'c ) 2 feet', 'd ) 3 feet', 'e ) 4 feet']
d