Elements, compounds and mixtures — practice question

Which statement gives the description of clean, dry air?

• AIt is a compound containing about $78\%$ nitrogen and $21\%$ oxygen only.
• BIt is a mixture of about $21\%$ nitrogen and $78\%$ oxygen only.
• CIt is a mixture of several gases, including nitrogen and oxygen.
• DIt is a compound containing nitrogen, oxygen, carbon dioxide and other gases.