Argon, Ar Oxygen, O2 Water, H2O Nitrogen, N2
I gave my students this multiple choice question: Air is composed of many different substances. Which of the following components of air is a compound rather than an element?

Only one answer is correct among these 4 choices:
- Argon, Ar
- Oxygen, O2
- Water, H2O
- Nitrogen, N2

Could you tell me which one is correct?

Water, H2O