Question

astronomers classify \gas\ as
and
these
two elements are the most
abundant in the universe.

Explanation:

Hydrogen and helium are the two most abundant elements in the universe. In astronomy, when referring to "gas" in celestial contexts (like in stars, gas giants, nebulae etc.), these two elements are commonly what is being discussed. Hydrogen is the lightest and most abundant element. Helium is the second - lightest and also very abundant, especially in stars where hydrogen fusion produces helium.

Answer:

Hydrogen, Helium