Two rigid boxes containing different ideal gases are placed on a table. Box A contains one mole of nitrogen at temperature T₀, while Box B contains one mole of helium at temperature (7/3) T₀. 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 boxes). Then, the final temperature of the gases, T_f, in terms of T₀ is

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D.