TOEFL iBT Reading

Practice session

Passage: Coral reefs are built by tiny animals that live with photosynthetic algae inside their tissues. The algae provide sugars produced from sunlight, while the coral gives the algae shelter and access to waste nutrients. When ocean temperatures stay unusually high, corals may expel the algae, losing both color and a major food source. If stressful conditions continue, the weakened coral can die. Which choice best describes the organization of the passage?

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