The region in the first quadrant bounded by y = 3 squareroot x and the line x = 8 forms the base of a solid. Cross sections of the solid perpendicular to the x-axis are squares. For what value of k does the line x = k divide the solid into two solids of equal volume? (A) 4 (B) 4.138 (C) 5.278 (D) 16/3 (E) 6.4