In a chemical reaction, the law of conservation of mass states that the total mass of the reactants must equal the total mass of the products.
In this case, the scientist starts with 30 grams of calcium carbonate (CaCO3). When it decomposes, it produces carbon dioxide (CO2) and calcium oxide (CaO). Regardless of the amounts produced, the total mass of the products will equal the mass of the reactants.
Therefore, the total mass of the products will be 30 grams.
So the correct response is: 30 grams.