What is most important gas for origin of life?

Carbon is present in living organism in from of organic compounds. The carbon dioxide and carbon monoxide gases served as source of carbon for synthesis of organic compounds during origin of life. The primitive atmosphere of earth was reducing atmosphere with little or no oxygen but was rich in water vapour, carbon monoxide, carbon dioxide, nitrogen and hydrogen. The high temperature, UV light and lightening provided energy for chemical reactions between hydrogen and free oxygen, nitrogen and carbon to form, ammonia, methane and cyanides. Primitive atmosphere had water in form of superheated steam which condensed with lowering of earth temperature and wash down the simple compounds (ammonia, methane and cyanides) formed in atmosphere. Water served site for origin of life as it supports both polymerizing and hydrolysing reactions as well as buffers the high temperature owing to its high latent heat of evaporation. Formation of simple compounds from primitive gases in atmosphere and their polymerization in ocean was possible due to absence of oxygen. Presence of free oxygen would have immediately oxidized the monomers and polymers and thereby would not have allowed build up organic compounds in primitive ocean followed by their complex reactions to form first life form. This makes oxygen the most important element for origin of life because its presence in present day oxidizing atmosphere does not allow the origin of life as it occurred in primitive reducing atmosphere. Correct answer is B.