The rods of length $L_1$ and $L_2$ are made of materials whose coefficients of linear expansion are $\alpha_1$ and $\alpha_2$. If the difference between the two lengths is independent of temperature,then:

• A
  $(\frac{L_1}{L_2}) = (\frac{\alpha_1}{\alpha_2})$
• B
  $\frac{L_1}{L_2} = \frac{\alpha_2}{\alpha_1}$
• C
  $L_1^2 \alpha_1 = L_2^2 \alpha_2$
• D
  $\alpha_1^2 L_1 = \alpha_2^2 L_2$