Two wires of equal length, one of aluminium and the other of copper have the same resistance. Which of the two wires is lighter? Hence explain why aluminium wires are preferred for overhead power cables. (`rho_Al = 2.63 xx 10^-8 Omega m, rho_Cu = 1.72 xx 10^-8 Omega m`. Relative density of A1 = 2.7, of Cu = 8.9.)