Think about the reaction from the electron transport chain in the mitochondria. Only consider the forward direction (the reaction from left to right)
cytochrome c(Fe2+) + cytochrome a(Fe3+) ? cytochrome c(Fe3+) + cytochrome a(Fe2+)
Which of the substrates is in the oxidized form?
Which of the products is in the oxidized form?
What is the oxidation half reaction?