The two most common gases of the Earth's atmosphere, and their percentages, are

A. oxygen (O₂), 21%; and carbon dioxide (CO₂), 78%.
B. nitrogen (N₂), 21%; and oxygen (O₂), 78%.
C. nitrogen (N₂), 78%; and oxygen (O₂), 21%.
D. water (H₂O), 21%; and oxygen (O₂), 78%.