Two wires of equal diameters with resistivities ${\rho _1}$ and ${\rho _2}$ and lengths $x_1$ and $x_2$ are joined in series. The equivalent resistivity of the combination is

• A
  $\frac{{\rho _1}{x_1} + {\rho _2}{x_2}}{{{x_1} + {x_2}}}$
• B
  $\frac{{\rho _1}{x_2} + {\rho _2}{x_1}}{{{x_1} - {x_2}}}$
• C
  $\frac{{\rho _1}{x_2} + {\rho _2}{x_1}}{{{x_1} + {x_2}}}$
• D
  $\frac{{\rho _1}{x_1} + {\rho _2}{x_2}}{{\rho _1} + {\rho _2}}$