Use the equation to answer the question.

The correct response is:

**There are two atoms with mass numbers of 1.**

In the reaction you provided (21H + 21H → 42H + energy), each "21H" represents a hydrogen isotope with a mass number of 2. However, if we look closely, the question is about the mass numbers before the reaction, and there are indeed two atoms (21H and 21H) that each have a mass number of 2.

So, the ideal phrasing should reflect that the reaction involves two atoms with mass numbers of 2, aligning with the formation of the product with a mass number of 4 (42H). Thus, a more precise statement would be:

**There are two atoms with mass numbers of 2.**

This indicates that the reaction starts with two deuterium nuclei (hydrogen isotopes with mass number 2).