The key to this question is making sure that both answers in your final choice meet both criteria - that is they must both be IMPORTANT and EXIST IN SMALL QUANTITIES. Nitrogen is the most abundant gas in our atmosphere so it does NOT exist in small quantities. This fact eliminates answers (a) and (c). Of the remaining two answers, only three chemicals are included. One of them is Argon which is the third most common gas in the atmosphere and certainly is not an important one as it does not react with anything. Clearly, water and ozone fit the bill best and so the answer is (b).