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Two wires of equal diameters, of resistivities ${\rho _1}$ and ${\rho _2}$ and lengths $l_1$ and $l_2$, respectively, are joined in series. The equivalent resistivity of the combination is
  • A$\frac{{{\rho _1}{l_1} + {\rho _2}{l_2}}}{{{l_1} + {l_2}}}$
  • B$\frac{{{\rho _1}{l_2} + {\rho _2}{l_1}}}{{{l_1} - {l_2}}}$
  • C$\frac{{{\rho _1}{l_2} + {\rho _2}{l_1}}}{{{l_1} + {l_2}}}$
  • D$\frac{{{\rho _1}{l_1} - {\rho _2}{l_2}}}{{{l_1} - {l_2}}}$
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