$\;l_{1}\;$ and $\;l_{2}\;$ are the lengths of two rods at $\;0^{0}C\;$ and have coefficients of linear expansion $\;\alpha_{1}\;$ and $\;\alpha_{2}\;$ respectively . If the difference between the lengths X is the same at all temperatures then $\;l_{1}\;$ is

Question

$(a)\;\large\frac{\alpha_{2}\;x}{\alpha_{1} -\alpha_{2}}\qquad(b)\;\large\frac{\alpha_{1}\;x}{\alpha_{1} -\alpha_{2}}\qquad(c)\;\large\frac{\alpha_{2}}{\alpha_{1}}\;x\qquad(d)\;\large\frac{\alpha_{1}}{\alpha_{2}}$