21. A hollow sphere of inner radius 8.0 cm and outer radius 9.0 cm floats half-submerged in a liquid of density 800kg/m3 . What is the density of the material of which the sphere is made?
(A)1.1×103kg/m3 
(B)1.3×10^{3kg/m3
(C)1.8×103kg/m3 
(D)2.4×103kg/m3 
(E)4.8×103kg/m3}