Concept:A paragraph must have a logical flow: introduce the main idea, provide background, give an example, state the consequence.Explanation:Sentence 3 introduces the definition of keystone species. Sentence 2 gives the origin of the concept. Sentence 5 explains its impact on ecology. Sentence 4 provides a specific example (sea otters). Sentence 1 concludes with the consequence of losing keystone species. The correct order is 3 → 2 → 5 → 4 → 1. Option B matches this sequence. Other options break the logical progression.Answer:B