AP BIOLOGY

Detailed explanation-1: -According to them primitive atmosphere was reducing atmosphere because hydrogen atoms (most numerous and most reactive) combined with all available oxygen atoms to form water and leaving no free oxygen. Therefore oxygen was not present in the primitive atmosphere of the earth.

Detailed explanation-2: -So, the correct answer is ‘Oxygen’.

Detailed explanation-3: -Oxygen is produced by plants by the process of photosynthesis, earlier there were no plants and vegetation, which’s why no oxygen in the early atmosphere.

Detailed explanation-4: -Considering the atmosphere of the primitive earth to have contained water vapor, carbon dioxide, carbon monoxide, methane, and ammonia and/or nitrogen, then in the presence of an energy source, such as sunlight or lightning, a number of small molecules like hydrogen cyanide, HCN, are formed.

Detailed explanation-5: -Hydrogen gas is not present in air, whereas Air is 78% nitrogen and 21% oxygen, with the remainder being trace gases like carbon dioxide, water vapour etc.