helium (He) hydrochloric acid (HCl) hydrogen (H2) nitrogen (N2)
I gave my students this multiple choice question: Which chemical is a compound?

Only one answer is correct among these 4 choices:
- helium (He)
- hydrochloric acid (HCl)
- hydrogen (H2)
- nitrogen (N2)

Could you tell me which one is correct?

hydrochloric acid (HCl)