$A$ copper rod of length $l_1$ and an iron rod of length $l_2$ are always maintained at the same common temperature $T$. If the difference $(l_2 - l_1)$ is $15\,cm$ and is independent of the value of $T$,then $l_1$ and $l_2$ have the values (given the linear coefficients of expansion for copper and iron are $\alpha_c = 2.0 \times 10^{-6}\,^{\circ}C^{-1}$ and $\alpha_i = 1.0 \times 10^{-6}\,^{\circ}C^{-1}$ respectively).

• A
  $l_1 = 15\,cm, l_2 = 30\,cm$
• B
  $l_1 = 30\,cm, l_2 = 15\,cm$
• C
  $l_1 = 10\,cm, l_2 = 25\,cm$
• D
  $l_1 = 25\,cm, l_2 = 10\,cm$