Câu hỏi:

Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question

Coral reefs are some of the most diverse ecosystems in the world, housing tens of thousands of marine species. With their hardened surfaces, corals are sometimes mistaken as being rocks. And, because they are attached, “taking root” to the seafloor, they are often mistaken for plants. However, unlike rocks, corals are alive. And unlike plants, corals do not make their own food. Corals are in fact animals. Appearing as solitary forms in the fossil record more than 400 million years ago, corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years.

Coral reefs are the largest structures on earth of biological origin. Coral reefs are unique and complex systems. Rivaling old growth forests in longevity of their ecological communities, well-developed reefs reflect thousands of years of history.

Corals are ancient animals related to jellyfish and anemones. The branch or mound that we often call “a coral” is actually made up of thousands of tiny animals called polyps. A coral polyp is an invertebrate that can be no bigger than a pinhead to up to 30 centimeters (1 foot) in diameter. The polyps extend their tentacles at night to sting and ingest tiny organisms called plankton and other small creatures. Each polyp has a saclike body and a mouth that is encircled by stinging tentacles. The polyp uses calcium carbonate (limestone) from seawater to build a hard, cup-shaped skeleton. This skeleton protects the soft, delicate body of the polyp.

Reefs only occur in shallow areas that are reachable by sunlight because of the relationship between coral and algae. Various types of microscopic algae, known as Symbiodinium, live inside of the coral, providing them with food and helping them to grow faster.

There are over 2,500 kinds (species) of corals. About 1,000 are the hard corals that build coral reefs. Other corals are soft corals. Soft corals have skeletons that are flexible and can bend with the water. The three main types of coral reefs are fringing reefs, barrier reefs, and coral atolls. The most common type of reef is the fringing reef. This type of reef grows seaward directly from the shore. They form borders along the shoreline and surrounding islands. When a fringing reef continues to grow upward from a volcanic island that has sunk entirely below sea level, an atoll is formed. Atolls are usually circular or oval in shape, with an open lagoon in the center. Any reef that is called a barrier reef gets its name because its presence protects the shallow waters along the shore from the open sea. That protection promotes the survival of many types of sea plant and animal life.

(Adapted from http://justfunfacts.com)

As mentioned in the last paragraph, why a reef is called a barrier reef?