Atom A is a reducing agent. Why? Well, because it reduces atom B. It makes this atom more negative. So we can also look at atom B as an oxidizing agent, because atom D takes away that electron from A, and it oxidizes A, and that's why it's the oxidizing agent. So Oxidation, or the loss of electrons and reduction or the gain of electron always comes in a pair, the same way that acids are always paired with the base.