A cylindrical buoy has a volume of 0.40m3 and unknown mass. It floats upright in river water (ρ=1000kgm−3). When an additional 200N load is placed on top, the buoy just submerges so that its upper surface is level with the water. Taking g=9.8ms−2, what is the mass of the unloaded buoy?
(a) 200kg
(b) 420kg
(c) 380kg
(d) 400kg