Which two gases primarily compose a nebula?

A nebula, which is a vast cloud of gas and dust in space, is primarily composed of hydrogen and helium. These two elements are the most abundant in the universe, making up about 98% of the total mass of a typical nebula.

Hydrogen, being the simplest and most abundant element, serves as the primary building block for stars and galaxies. Helium, the second most abundant element, is produced through nuclear fusion within stars. The presence of these gases is crucial as they contribute to the formation of stars and planetary systems over time.

The other combinations mentioned do not accurately represent the dominant composition of nebulae. Oxygen and nitrogen, while present in smaller amounts, are not significant contributors to the overall mass of a nebula. Similarly, the presence of helium and carbon would not sufficiently capture the essential components that define the majority of these celestial structures. In summary, hydrogen and helium are the fundamental elements that form the backbone of nebulae, making option B the correct answer.