Corals belong to a group of animals called cnidarians. This group encompasses hard andsoftcorals, sea fans, gorgonians, hydroids,jellyfish, and sea anemones. Although the group isremarkablydiverse, there are a few features shared by all. They have a free-swimminglarvalstageand a simple body plan: a central mouth,throughwhichmaterialpasses in and out of the body, and a ring of tentacles. The other distinguishing feature of the cnidarians is the presence of nematocysts (stingingcells), which are used to catchprey.

Individualcorals are known ascoralpolyps; each is a single animal. In isolation acoralpolyplooks verysimilarto a sea anemone, but unlike sea anemones, which live separated from each other, corals form colonies. A fewspeciesremainsolitary, but in most cases, new polypsbudoff theinitialfoundingpolyp, andgraduallycolonies of thousands orevenmillions of polyps will grow, each connected to its neighbors by livingtissue. Freed from the limitations of living alone, colonies can grow toimmensesizes and live a very long time.

Corals come in all shapes and sizes, but thebasicplan is the same; polyps live as a surfacelayeron somesortof structure, be it hard and inflexible or rubbery. The hard, or true corals, for which reefs are most famous, buildlimestoneskeletonsbeneaththe livingtissue. Softcorals, as their name implies, do not formsolidskeletons;insteadtheysecretelimestonecrystalstructures called sclerites, which are embeddedwithina jellylike matrixbeneaththe polyps. When they die, little is left of mostsoftcorals as thelimestoneelementof their makeup is so small and easily breaks up.

Bycontrast, some sea whips and fans and,notably, the black corals, have suchdenseskeletons oflimestone,protein, and minerals that they are highlydurableand are collected and polished forjewelry. Theskeletonof black corals is moredensethe slower it grows, and it grows more slowly at depth. So it is the very deep, older colonies that are most highlyprize.

Coralreefs would not exist if it were not for theabilityofcoralpolyps tosecretelimestoneorcalciumcarbonate. Sea water surrounding acoralis very rich in dissolvedcalciumcarbonate, but thefluidinside thepolypcannotretaina largequantityofcalciumcarbonate, so it is laid down asmicroscopicneedle-shaped crystalsbeneathand around thepolyp. Thisprocessoccurs in two stages. As thepolypexpands to feed with its tentacles at night, it lifts off theskeleton, rather like a glove coming part way off a hand. At thisstage, thecalciumcarbonatecrystals form ridges. During the following day (when thecoralpolypis retracted and lying on its new structure), the valleys between the ridges fill in with morecalciumcarbonateand theskeletontakes on a smootherappearance. Because theskeletonof hard corals is made oflimestoneorcalciumcarbonate, it ispurewhite.

Eachcoralspecieslays down itsskeletonin a different way. This gives rise to theextraordinaryrange of shapes and forms that hard corals take. Some form large boulders, where the polyps live in small,isolateddepressions or grooves in theskeleton. Some grow in branches, which can be small andstubby, while others are spreading, treelike structures. Still others growdelicate, leaflike plates or flat tables. The range is huge, and to complicate the matter further, the samespecieswill grow in a different way depending on thephysicalcharacteristics of the place in which it finds itself. Although acoral’sgeneticblueprint isfundamentalin determining what it looks like, itsappearanceis also affected by waves, currents, light, and competition forspaceon thereef. Such variations incoralform havecomplicatedthe matter ofidentificationover the years.

Questions:
question_stem: According to paragraph 1, which of the following is true about the group of animals called cnidarians?
choices: tag: A
text: Its members are remarkably similar in appearance.

tag: B
text: They are unable to catch their own food.

tag: C
text: They can swim in their earliest stages of life.

tag: D
text: They have highly complex bodies.
answer_key: C
explanation: 题干问“关于 cnidarians 哪个表述正确”整段都是对于这个物种的信息描述，逐句核对；C 选项对应原文第四句话“they have a free-swimming larval stage ”幼虫阶段对应 C 选项的 earliest stages of life，故为正确答案。A 中的 similar appearance 和原段第三句 diverse 相矛盾，不选，B 选项 unable 和最后一句 catch food 矛盾，不选；D 选项 complex body 和第三句 simple body 矛盾，不选。

question_stem: According to paragraph 2, what do corals and sea anemones have in common?
choices: tag: A
text: The individuals of both species resemble each other.

tag: B
text: They both form large colonies.

tag: C
text: The individuals of both species are called polyps.

tag: D
text: They both live very long lives.
answer_key: A
explanation: 题干提问“corals and sea anemones 的共性”，定位原文第二句，In isolation a coral polyp looks very similar to a sea anemone, 符合 A 选项 resemble each other，故 A 为正确答案。B 选项 colony 只是 coral，C 选项 polyps 是 coral，故都不选，D 选项对应原文最后一句话，只有 coral 的 colony 才 live long，故也不选。

question_stem: Which of the following can be inferred about coral reefs from paragraph 3?
choices: tag: A
text: They are only found on hard structures.

tag: B
text: The most famous ones are supported by solid skeletons.

tag: C
text: They require sclerites to live.

tag: D
text: The jellylike matrix within them contains no limestone.
answer_key: B
explanation: 题干提问“关于 coral reefs 可以推断出什么”，定位到原文第二句话，The hard, or true corals, for which reefs are most famous， 故选 B。A 选项 only on hard，太极端，原文说最出名，不选；D 选项原文只说 limestone 含量低，但没有到完全否定，不选；C 选项他们依靠 sclerites 为生，原文没提到他们之间的关系，故不选。

question_stem: The author mentions sea whips, fans, and black corals in paragraph 4 as examples of
choices: tag: A
text: cnidarians that are especially difficult to find

tag: B
text: cnidarians whose skeletons do not easily break up

tag: C
text: the many different kinds of corals that exist

tag: D
text: reefs formed by soft corals
answer_key: B
explanation: 题干提问“ The author mentions sea whips, fans, and black corals in paragraph 4 as examples of”定位到第一句话 have such dense skeletons of limestone, that they are highly durable，对应 B 选项 do not easily break up，故 B 为正确答案。ACD 均未提及，不选。

question_stem: According to paragraphs 3 and 4, the corals considered to be the most valuable are those that
choices: tag: A
text: secrete limestone crystal structures

tag: B
text: attach themselves to rubbery surfaces

tag: C
text: contain little limestone

tag: D
text: grow in the deepest waters
answer_key: D
explanation: 题干提问“什么样的珊瑚被认为是最珍贵的”地位第四段最后一句，So it is the very deep, older colonies that are most highly prized，故 D grow in the deepest waters，正确。ABC 均不符合题目定位，故不选。

question_stem: 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.
choices: tag: A
text: Calcium carbonate from surrounding sea water is stored inside a coral polyp in the form of small microscopic-shaped crystals.

tag: B
text: Since only a small amount of calcium carbonate can be stored inside a coral polyp, it is also stored outside it.

tag: C
text: The fluid inside a coral polyp is richer in calcium carbonate than the surrounding crystals containing calcium carbonate.

tag: D
text: Calcium carbonate does not exist in crystal form in seawater, but it is found in that form inside coral polyps.
answer_key: B
explanation: 高亮句中有转折，有因果，故重点理解 but 之后信息，以及区分因果逻辑。因为 the fluid inside the polyp 不能保留大量的 calcium carbonate, 所以在 polyp 的地下和周围分布.故 B 为正确答案，信息对，因果关系也对应；A Calcium carbonateis stored inside a coral polyp 和原文矛盾，错，C 内部比周围含有更多的 calcium carbonate 跟原句 the fluid inside the polyp cannot retain a large quantity of calcium carbonate 相反，错；D does not exist 和原文不符，错。

question_stem: The author mentions a glove in paragraph 5 in order to explain
choices: tag: A
text: how many tentacles polyps have

tag: B
text: how polyps feed

tag: C
text: how polyps protect themselves

tag: D
text: why corals are beautiful
answer_key: B
explanation: 题干问“作者说 glove 的目的是什么”，根据 glove 定位到第四句话，读整句，了解到说 polyp 的捕食行为，用触须在夜晚捕食，它把骨架卸掉，就像把手套从手上脱下一部分。故选 B，表达饮食的方式。A.说触须的数量，原文没提，错；C. polyps 如何保护自己，原文没提，错；D.说为什么是美丽的，错误。

question_stem: The phrase “gives rise to” in the passage is closest in meaning to
choices: tag: A
text: alters

tag: B
text: causes

tag: C
text: increases

tag: D
text: builds on
answer_key: B
explanation: gives rise to 造成; 引起; 导致; 故选 B cause 导致
 A. alters  改变 C. increases 增加   D. builds on 以..为基础，不符合词义，故 ACD 不选。