30. a co-axial cable of uniform cross section contains an insulating material of dielectric constant 3.5. the radius of the central wire is 0.01m and that of th

30. A co-axial cable of uniform cross section contains an insulating material of dielectric constant 3.5. The radius of the central wire is 0.01m and that of the sheath is 0.02 m. The capacitance per kilometer of a cable is (A) 280.5 nF (B) 28.05 nF (C) 56.10 nF (D) 2.805 nF