Here's why:

* Chemistry is the study of matter and its properties, as well as how matter changes.

* Hydrogen and oxygen are elements, the fundamental building blocks of matter.

* Chemists study the properties of these elements, how they react with each other, and how they form compounds like water (H₂O).

However, depending on the specific area of research, the scientist could also be:

* A physicist: If they focus on the fundamental physics of hydrogen and oxygen atoms, like their energy levels or interactions with light.

* An atmospheric scientist: If they study the role of hydrogen and oxygen in the Earth's atmosphere.

* A materials scientist: If they explore how hydrogen and oxygen contribute to the properties of different materials.

So, while a chemist is the most likely answer, the specific area of expertise would depend on the specific research focus.