Correct Answer: (a) Nitrogen and oxygen
Solution:The atmosphere is a mixture of different gases, of which 99% are mainly Nitrogen (78.084%) and Oxygen (20.947%). Remaining approximately 1% are Argon, Carbon dioxide, Neon, Helium, Methane, Hydrogen, Nitrous Oxide, Ozone, Krypton, Xenon, etc. Heavy gases are found at the bottom of atmosphere and light gases are found at the top. Carbon dioxide is found up to 20 km above Earth, Oxygen and nitrogen up to 100 km, Hydrogen up to 125 km and except Ozone other gases are found at much higher elevation