Human gas exchange — practice question

The table gives the percentages of four gases in inspired air and expired air. Which gas is carbon dioxide?

• AInspired air: $0.04$, expired air: $4.14$
• BInspired air: $0.50$, expired air: $2.00$
• CInspired air: $20.80$, expired air: $12.20$
• DInspired air: $78.50$, expired air: $78.50$