A vessel contains oil (density = 0.8 g cm–3) over mercury (density = 13.6 g cm–3). A uniform sphere floats with half its volume immersed in mercury and the other half in oil. The density of the material of sphere in g cm–3 is :–
(a) 3.3
(b) 6.4
(c) 7.2
(d) 12.8