UPSC Prelims 2023— Question 55

This question tests Biofilters in Recirculating Aquaculture under Aquaculture Technology. The central point is Biofilters convert ammonia to nitrate in recirculating aquaculture; they do not increase phosphorus as a nutrient. Option A: Only one. This distractor is not the keyed answer because it conflicts with the specific factual relationship, chronology, institutional role, geographic association or conceptual distinction tested in the question. Option B: Only two. This is the keyed answer because it matches the governing fact or concept: Biofilters convert ammonia to nitrate in recirculating aquaculture; they do not increase phosphorus as a nutrient. Option C: All three. This distractor is not the keyed answer because it conflicts with the specific factual relationship, chronology, institutional role, geographic association or conceptual distinction tested in the question. Option D: None. This distractor is not the keyed answer because it conflicts with the specific factual relationship, chronology, institutional role, geographic association or conceptual distinction tested in the question. The question should be approached by checking each statement independently rather than accepting a familiar phrase at face value. This is particularly important in UPSC questions where a single incorrect institutional attribution, location, chronology, absolute word or causal link can invalidate an otherwise plausible option. The broader revision takeaway is to connect the keyed answer with the underlying concept rather than memorizing only the letter. The options also illustrate how UPSC constructs distractors by mixing correct facts with one altered detail. In revision, candidates should therefore retain both the positive fact and the nearest contrasting fact so that the same concept can be recognized when the framing changes in another year.