The correct answer is A. Some of the atoms in the paper became gases.
When paper burns, it undergoes a chemical reaction known as combustion, where it reacts with oxygen in the air. This reaction produces gases (such as carbon dioxide, water vapor, and other compounds) as byproducts, which escape into the atmosphere. As a result, the mass of the ashes left behind is less than that of the original paper. Option B is incorrect because atoms cannot be destroyed; they are merely rearranged during the chemical reaction. Option C is incorrect because the ashes do not come from new atoms but rather from the original atoms, and option D is misleading, as no additional atoms combine with the paper; rather, atoms are lost as gases during combustion.