Two wires of equal diameters, of resistivities ${\rho }_1$ and ${\rho }_2$ and length $l_1$ and $l_2$, respectively are joined in series. The equivalent resistivity of the combination is

• Correct answer
  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

  $\frac{{\rho }_1{l}_1+{\rho }_2{l}_2}{l_1-l_2}$

Hence, the correct answer is option: 'A'.