托福阅读official66 Which of the sentences below best expresses the essential information in the highlighted sentence in答案解析【小站备考】

Corals belong to a group of animals called cnidarians. This group encompasses hard and soft corals, sea fans, gorgonians, hydroids, jellyfish, and sea anemones. Although the group is remarkably diverse, there are a few features shared by all. They have a free-swimming larval stage and a simple body plan: a central mouth, through which material passes in and out of the body, and a ring of tentacles. The other distinguishing feature of the cnidarians is the presence of nematocysts (stinging cells), which are used to catch prey.

Individual corals are known as coral polyps; each is a single animal. In isolation a coral polyp looks very similar to a sea anemone, but unlike sea anemones, which live separated from each other, corals form colonies. A few species remain solitary, but in most cases, new polyps bud off the initial founding polyp, and gradually colonies of thousands or even millions of polyps will grow, each connected to its neighbors by living tissue. Freed from the limitations of living alone, colonies can grow to immense sizes and live a very long time.

Corals come in all shapes and sizes, but the basic plan is the same; polyps live as a surface layer on some sort of structure, be it hard and inflexible or rubbery. The hard, or true corals, for which reefs are most famous, build limestone skeletons beneath the living tissue. Soft corals, as their name implies, do not form solid skeletons; instead they secrete limestone crystal structures called sclerites, which are embedded within a jellylike matrix beneath the polyps. When they die, little is left of most soft corals as the limestone element of their makeup is so small and easily breaks up.

By contrast, some sea whips and fans and, notably, the black corals, have such dense skeletons of limestone, protein, and minerals that they are highly durable and are collected and polished for jewelry. The skeleton of black corals is more dense the slower it grows, and it grows more slowly at depth. So it is the very deep, older colonies that are most highly prized.

Coral reefs would not exist if it were not for the ability of coral polyps to secrete limestone or calcium carbonate. Sea water surrounding a coral is very rich in dissolved calcium carbonate, but the fluid inside the polyp cannot retain a large quantity of calcium carbonate, so it is laid down as microscopic needle-shaped crystals beneath and around the polyp. This process occurs in two stages. As the polyp expands to feed with its tentacles at night, it lifts off the skeleton, rather like a glove coming part way off a hand. At this stage, the calcium carbonate crystals form ridges. During the following day (when the coral polyp is retracted and lying on its new structure), the valleys between the ridges fill in with more calcium carbonate and the skeleton takes on a smoother appearance. Because the skeleton of hard corals is made of limestone or calcium carbonate, it is pure white.

Each coral species lays down its skeleton in a different way. This gives rise to the extraordinary range of shapes and forms that hard corals take. Some form large boulders, where the polyps live in small, isolated depressions or grooves in the skeleton. Some grow in branches, which can be small and stubby, while others are spreading, treelike structures. Still others grow delicate, leaflike plates or flat tables. The range is huge, and to complicate the matter further, the same species will grow in a different way depending on the physical characteristics of the place in which it finds itself. Although a coral’s genetic blueprint is fundamental in determining what it looks like, its appearance is also affected by waves, currents, light, and competition for space on the reef. Such variations in coral form have complicated the matter of identification over the years.

OFFICIAL66 Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

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