Two wires of equal diameters of resistivities $\rho_1$ and $\rho_2$ and lengths $x_1$ and $x_2$ respectively are joined in series. The equivalent resistivity of the combination is :

Question

( A ) $\frac{\rho_1 x_1 - \rho_2 x_2}{x_1 - x_2}$
( B ) $\frac{\rho_1 x_1 + \rho_2 x_2}{x_1 + x_2}$
( C ) $\frac{\rho_1 x_2 + \rho_2 x_1}{x_1 + x_2}$
( D ) $\frac{\rho_1 x_2 - \rho_2 x_1}{x_1 - x_2}$