UPSC Prelims 2021— Question 77

The phosphorus cycle is distinctive because its major natural reservoir is phosphate-bearing rocks and sediments, and weathering releases phosphate into soils and aquatic systems. The nitrogen cycle is strongly linked to atmospheric nitrogen and biological fixation, while carbon and sulphur have major atmospheric and geological pathways. Therefore phosphorus is the correct cycle. The final answer-key position for Set C identifies option C as the correct option. Option A — Carbon cycle — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. Option B — Nitrogen cycle — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. Option C — Phosphorus cycle — is correct because it is the alternative that matches the constitutional, historical, geographical, scientific or economic principle established above. Option D — Sulphur cycle — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. For PYQ interpretation, the distinction between the correct alternative and merely related facts is important. UPSC often places plausible distractors beside the precise statement that matches the underlying rule, chronology, definition or empirical relationship. The answer should therefore be read against the exact wording of the question rather than against a broadly related fact.