Explanation

Option C is correct because soil generally has less oxygen and more carbon dioxide compared to the atmosphere. Option A is incorrect because while soil has less oxygen than 10 atmospheres, it also has higher CO2 levels making 'Both a & b' better. Option B is incorrect because it only mentions CO2 and ignores the oxygen comparison needed for option C. Option D is incorrect because both statements (a and b) are generally true about soil composition compared to the atmosphere.