It should beobviousthat cetaceans—whales, porpoises, and dolphins—are mammals.

They breathethroughlungs, notthroughgills, and give birth to live young. Theirstreamlinedbodies, theabsenceofhindlegs, and the presence of a fluke and blowhole cannotdisguisetheir affinities with landdwellingmammals. However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs arefunctionalboth on land and at sea),

it is not easy toenvisionwhat the first whales looked like. Extinctbut already fullymarinecetaceans are known from thefossilrecord. How was thegapbetween a walkingmammaland a swimming whale bridged? Missing until recently were fossils clearlyintermediate, or transitional, between land mammals and cetaceans.

Veryexcitingdiscoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldestfossilwhale. Thefossilwas officially named Pakicetus in honor of the country where the discovery was made. Pakicetus was found embedded in rocks formed from river deposits that were 52 million years old. The river that formed these deposits was actually not far from anancientocean known as the Tethys Sea.

Thefossilconsists of a completeskullof an archaeocyte, anextinctgroup of ancestors of modern cetaceans. Although limited to askull, the Pakicetusfossilprovidespreciousdetails on the origins of cetaceans. Theskullis cetacean-like but its jawboneslackthe enlargedspacethat is filled with fat or oil and used for receiving underwatersoundin modern whales. Pakicetus probably detectedsoundthroughthe ear opening as in land mammals. Theskullalso lacks a blowhole, anothercetaceanadaptationfor diving. Other features, however, show experts that Pakicetus is a transitional form between a group ofextinctflesh-eating mammals, the mesonychids, and cetaceans. It has been suggested that Pakicetus fed on fish inshallowwater and was not yet adapted for life in the open ocean. It probably bred and gave birth on land.

Anothermajordiscovery was made in Egypt in 1989. Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and nowexposedin the Saharadesert. This whale lived around 40 million years ago, 12 million years after Pakicetus. Many incomplete skeletons were found but they included, for the first time in an archaeocyte, a completehindleg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long Basilosaurus on land. Basilosaurus was undoubtedly a fullymarinewhale with possibly nonfunctional, or vestigial,hindlegs.

Anevenmoreexcitingfind was reported in 1994, also from Pakistan. The nowextinctwhale Ambulocetus natans ("the walking whale that swam") lived in the Tethys Sea 49 million years ago. It lived around 3 million years after Pakicetus but 9 million before Basilosaurus. Thefossilluckily includes a goodportionof thehindlegs. The legs were strong and ended in long feet very much like those of a modern pinniped. The legs were certainlyfunctionalboth on land and at sea. The whale retained a tail and lacked a fluke, themajormeans oflocomotionin modern cetaceans. The structure of thebackboneshows, however, that Ambulocetus swam like modern whales by moving therearportionof its body up and down,eventhough a fluke was missing. The largehindlegs were used forpropulsionin water. On land, where it probably bred and gave birth, Ambulocetus may have moved around very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.

Questions:
question_stem: In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?
choices: tag: A
text: It clearly indicates that cetaceans are mammals.

tag: B
text: It cannot conceal the fact that cetaceans are mammals.

tag: C
text: It is the main difference between cetaceans and land-dwelling mammals.

tag: D
text: It cannot yield clues about the origins of cetaceans.
answer_key: B
explanation: 根据 blowhole 查找原文，找到：the presence of a fluke1 and blowhole2 cannot disguise their affinities with land dwelling mammals.选项 B 正确，是对该句的同义改写。
A 错误，反了。
C 错误，没有提到。
D 错误，没有提到。

question_stem: Which of the following can be inferred from paragraph 1 about early sea otters?
choices: tag: A
text: It is not difficult to imagine what they looked like.

tag: B
text: There were great numbers of them.

tag: C
text: They lived in the sea only.

tag: D
text: They did not leave many fossil remains.
answer_key: A
explanation: 典型取非推理题。根据 sea otters 定位原文，找到：unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like。A 正确。BCD 都不能推出。

question_stem: The word “precious” in the passage is closest in meaning to
choices: tag: A
text: exact

tag: B
text: scarce

tag: C
text: valuable

tag: D
text: initial
answer_key: C
explanation: Precious: 宝贵的，珍贵的。和选项 C 的 valuable 呼应。A 是精准的；B 是稀少的；D 是起初的。对应原文 Although limited to a skull, the Pakicetus fossil provides precious details on the origins of cetaceans.原句中有明显的转折逻辑，转折前强调化石的有限性，转折之后说这些化石提供了 precious details。从转折关系可以推断，precious 应该是正面描述，因为转折前是负面描述，这样才可以形成合理的转折关系。那么 BD 都可以直接排除，都是中性词，而 AC 都有可能。C 更接近 precious，所以正确。

question_stem: Pakicetus and modern cetaceans have similar
choices: tag: A
text: hearing structures

tag: B
text: adaptations for diving

tag: C
text: skull shapes

tag: D
text: breeding locations
answer_key: C
explanation: 整段大部分内容都在描述两者的差异，只有第三句话描述到了相似之处： The skull is cetacean-like but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving underwater sound in modern whales.这一句说 P 的头盖骨是 cetacen-like，这就对应了 C 选项。所以正确答案是 C。

question_stem: The word “It” in the passage refers to
choices: tag: A
text: Pakicetus

tag: B
text: fish

tag: C
text: life

tag: D
text: ocean
answer_key: A
explanation: 指代题，根据前句，It has been suggested that Pakicetus fed on fish in shallow water and was not yet adapted for life in the open ocean。说的就是 P，所以选 A

question_stem: The word “exposed” in the passage is closest in meaning to
choices: tag: A
text: explained

tag: B
text: visible

tag: C
text: identified

tag: D
text: located
answer_key: B
explanation: exposed: 暴露的。和选项 B 呼应。A 是解释的；C 是核实的；D 是定位的。对应原文 Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and now exposed in the Sahara desert.这句话说发现了另一种鲸鱼的 skeleton，那么 exposed 兴衔接这个语境，应该也是说沙漠里能够看到这些 skeleton，B 就最为合适了。

question_stem: The hind leg of Basilosaurus was a significant find because it showed that Basilosaurus
choices: tag: A
text: lived later than Ambulocetus natans

tag: B
text: lived at the same time as Pakicetus

tag: C
text: was able to swim well

tag: D
text: could not have walked on land
answer_key: D
explanation: 用 hind leg 定位到第四句，但这句话没有说出重要性，往下看，such 告诉你下一句也是相关内容： Such legs would have been far too small to have supported the 50-foot-long Basilosaurus on land.这句话中强调了这个腿太小了以至于不恩能够在陆地上支撑，对应 D 选项。

question_stem: It can be inferred that Basilosaurus bred and gave birth in which of the following locations
choices: tag: A
text: On land

tag: B
text: Both on land and at sea

tag: C
text: In shallow water

tag: D
text: In a marine environment
answer_key: D
explanation: 根据 Basilosaurus 定位在4段，or the first time in an archaeocyte, a complete hind leg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long Basilosaurus on land. D 选项是同义改写，正确。ABC 明显错误。

question_stem: Why does the author use the word “luckily” in mentioning that the Ambulocetus natans fossil included hind legs?
choices: tag: A
text: Fossil legs of early whales are a rare find.

tag: B
text: The legs provided important information about the evolution of cetaceans.

tag: C
text: The discovery allowed scientists to reconstruct a complete skeleton of the whale.

tag: D
text: Until that time, only the front legs of early whales had been discovered.
answer_key: B
explanation: luckily 所在句，和后面两句话都是在描述 hing leg 的发现的意义，连起来读找到他所支撑的主旨句，即这一段的的第六句话： The fossil luckily includes a good portion of the hind legs. The legs were strong and ended in long feet very much like those of a modern pinniped. The legs were certainly functional both on land and at sea. luckily 意为"幸运地"，称其"幸运"是就 A 化石的科学价值而言：因为它具有 a good portion of the hind leg(比例完整的后肢)，从而（在第六句中）使科学家推出"The legs were certainly functional both on land and at sea"（A 动物为水陆两栖）。综上，正是由于 legs 提供了重要的科学信息，人们才称它 luckily，因此选择 B。

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: Even though Ambulocetus swam by moving its body up and down, it did not have a backbone.

tag: B
text: The backbone of Ambulocetus, which allowed it to swim, provides evidence of its missing fluke.

tag: C
text: Although Ambulocetus had no fluke, its backbone structure shows that it swam like modern whales.

tag: D
text: By moving the rear parts of their bodies up and down, modern whales swim in a different way from the way Ambulocetus swam.
answer_key: C
explanation: 这句话的主要逻辑是转折，有 even though 来分成2部分。尽管 fluke 缺失，Ambulocetus 像鲸鱼一样，靠身体后部的摆动来游动。C 选项完全对应，正确；A 选项的转折前后内容搞混；B 的 providing evidence 原文没提及；D 的 different way 和原句矛盾。

question_stem: The word “propulsion” in the passage is closest in meaning to
choices: tag: A
text: staying afloat

tag: B
text: changing direction

tag: C
text: decreasing weight

tag: D
text: moving forward
answer_key: D
explanation: propulsion: 推进。Pro 这个前缀有向前的意思，所以正确答案是 D。A 是保持漂浮；B 是改变方向；C 是减少重量。对应原文 The large hind legs were used for propulsion in water.这里说了大的后肢是用来在水中 propusion 的，这里4个选项带进去貌似都碎，但从 pro 这个前缀来判断，还是 D 最正确。