7D
By the book if player C was the only player with cards he would win the full amount, however I would generally invoke the 1st, returning player A's river bet as it sounds as if player C has made no action in this case, and therefore we can not be sure with any kind of certainty that he would of called the river bet anyway.
Fairest way would be to rule the 1st, return the river bet and award the rest of the pot to player c.
Regards
Stuart