Coral reefs are made up of which of the following?

Directions: Read the passage below and answer the question that follows.

Passage 1
Coral reefs are one of the Earth’s most beautiful, ancient and complex ecosystems. They play an essential role in sustaining life in the sea and serve as a source of food and protection for human communities. But coral reefs face an uncertain future. As a result of growing human and environmental assaults, reefs are among the most threatened ecosystems on earth. Ecological research provides information fundamental to understanding and combating this trend. Corals are actually animals with plants living inside them. A coral structure is actually composed of hundreds or thousands of tiny animals growing together as a colony which make a coral. Several polyps make a coral which together form a reef.
Coral reefs are the most diverse and productive marine communities on the planet. They play a critical role as habitat and nursery grounds for 10 to 20% of the world’s fisheries. They are intimately connected to other marine communities such as mangrove forests, sea grass beds, and the open seas as water currents transport larvae, plants, animals, nutrients, and organic materials. They play a significant role in the development of other ecosystems such as mangroves and wetlands and protect coastlines from wave and storm damage and erosion.