If two rods of length L and 2L having coefficients of linear expansion $\mathrm{\alpha } \mathrm{and} 2\mathrm{\alpha }$ respectively are connected so that total length becomes 3L. The average coefficient of linear expansion of the composite rod equals:

• A

  $\frac{3}{2}\mathrm{\alpha }$

• B

  $\frac{5}{2}\mathrm{\alpha }$

• C

  $\frac{5}{3}\mathrm{\alpha }$

• D

  None of these