Two rods having length l and 2l, made of materials with linear expansion coefficient $2\alpha$ and $\alpha$ are soldered together so as to form a single rod of length 3l. The equivalent coefficient of linear expansion for the composite rod is 

• A

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

• B

  $\raisebox{1ex}{$4\alpha $}\!\left/ \!\raisebox{-1ex}{$3$}\right.$

• C

  $\frac{3\alpha }{4}$

• D

  $\frac{4\alpha }{5}$