A copper rod of 2cm diameter is completely encased in a steel tube of inner diameter 2 cm and outer diameter 4cm. Under an axial load, the stress in the steel tube is 100 N/mm2. If $E_{s}=2E_{C}$ then the stress in the copper rod is
(a) 50 N/mm2 (b)33.33 N/mm2
(c)100 N/mm2 (d)300 N/mm2