Two rigid boxes containing different ideal gases are placed on a table. Box $A$ contains one mole of nitrogen at temperature $T_0$,while Box $B$ contains one mole of helium at temperature $(7/3)T_0$. The boxes are then put into thermal contact with each other,and heat flows between them until the gases reach a common final temperature (ignore the heat capacity of the boxes). The final temperature of the gases,$T_f$,in terms of $T_0$ is:

• A
  $T_f = \frac{5}{2}T_0$
• B
  $T_f = \frac{3}{7}T_0$
• C
  $T_f = \frac{7}{3}T_0$
• D
  $T_f = \frac{3}{2}T_0$