Substances A and B react in a ratio of A + B → AB. Colin was combining substance A and substance B to form substance AB. He added 10 g of substance A and 45 g of substance B.

In the reaction A + B → AB, the product contains all the mass of A and B that react. Add the amounts Colin combined: 10 g + 45 g = 55 g. Therefore, the mass of substance AB is 55 g, choice D.

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