Two boxes containing different ideal gases are placed on a table. Box $A$ contains $1 \text{ mole}$ of nitrogen gas at temperature $T_o$,and box $B$ contains $1 \text{ mole}$ of helium gas at temperature $(7/3) T_o$. If they are brought into thermal contact such that both gases reach a final common temperature $T_f$,what is the final temperature $T_f$ in terms of $T_o$? (Neglect the heat capacity of the boxes.)

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