1 The mysterious qualities of eels have captivated many scientists. In 1886, a French zoologist put a tiny leaf-shaped saltwater fish into a tank in his laboratory. The fish, about four centimeters long, startled the scientist when it turned into a tiny eel—long, rounded, and clear as window glass. Ten years later, another zoologist observed the same metamorphosis in the Mediterranean Sea and concluded that salt water was important to the process. In 1920, Danish biologist Johannes Schmidt tracked eels in the Atlantic Ocean, eventually finding individuals of the smallest size in the Sargasso Sea, an area of the North Atlantic between the West Indies and the Azores that is approximately 1,100 kilometers wide and 3,200 kilometers long and surrounded by four major ocean currents.
2 Atlantic eels have five life phases: larva, glass eel, elver, immature yellow eel, and mature silver eel. Eels are catadromous fish, living most of their lives in fresh water and then migrating to the sea to reproduce. Their migration is an epic life journey, north through the Atlantic, toward the shore, into rivers and lakes, and eventually back to the Sargasso Sea, swimming in the open water by night. Scientists have not yet fully documented adult eels arriving at the Sargasso, or eels mating or laying eggs there, but they have observed the newly hatched larvae clustering in the floating Sargassum weed. The tiny larvae rise to the surface, where they begin their journey by drifting on the Gulf Stream toward either North America or Europe. A year may pass before the young fish swim toward the shore and fresh water. As they approach the coast, the larvae transform into transparent glass eels, five to seven centimeters in length. It is not yet known how the eels find their way inland, but once glass eels leave the ocean and enter fresh water, they swim upstream, eventually turning into thin, black, ten-centimeter elvers.
3 As the elvers grow, they change into adolescent yellow eels, long and slender with brownish-yellow sides and bellies. Yellow eels spend the next several years in flesh water, living in rivers and lakes for up to twenty years. Nearly all of the eels in lakes are female, while those in rivers and estuaries are mainly male. As the adolescents mature into adults, their eyes grow larger, their backs darken, their sides become silver, and their bellies turn silver-white. They start changing into mature silver eels, ready to begin their southward migration to the Sargasso Sea in late autumn. The transformation into sexually mature fish occurs deep in the Atlantic, during the long swim back to their mating site.
4 During their long migration, eels face numerous hurdles. Most of these are the result of human activity, such as the dams, weirs, and other physical impediments that have compromised many river systems. Young black elvers require help getting past dams as they migrate upstream. Because eels are able to breathe through their skin as well as their gills, they can travel over wet surfaces on land, so in some locations biologists have devised wet ramps to assist their climb. During their downstream journey, silver eels can be killed when they swim into the turbines of hydroelectric dams, and a river"s entire eel run can be destroyed in just a few nights. To combat the problem, some energy companies slow the rotation of the blades during the migration so that the eels might pass safely through the turbines. Another hurdle to the eels" passage is the presence of ancient eel weirs, structures of various types built by every human culture that ever inhabited the region. Natural predators also take a toll: eagles, osprey, snapping turtles, and bass all feed on eels.
5 Overfishing and exploitive fisheries have caused eel populations to plummet. Fisheries target eels at three phases of their lives. Before the tiny glass eels can reach shore, thousands are trapped in nets for export to fish farms in Asia. Yellow eels are caught in eel pots for bait, and silver eels are caught in traps for specialty markets around the world. In addition to overfishing, eels will face a more serious threat if climate change alters the speed or direction of the currents around the Sargasso Sea. Even a slight shift in the currents would impact the mating and hatching territories of the Atlantic eel.

单选题 The word metamorphosis in paragraph 1 is closest in meaning to

单选题 What is the main purpose of paragraph 1?

单选题 According to the passage, one characteristic that defines catadromous fish is

单选题 Scientists have direct evidence of which eel behavior in the Sargasso Sea?

单选题 The word drifting in paragraph 2 is closest in meaning to

单选题 According to paragraph 2, there is scientific uncertainty about

单选题 One difference between eels living in rivers and those living in lakes is

单选题 Which sentence best expresses the essential information in the highlighted sentence in paragraph 4? Incorrect choices change the meaning in important ways or leave out essential information.

单选题 The word combat in paragraph 4 is closest in meaning to

单选题 Paragraph 4 supports which of the following statements about eel migration?

单选题 The word plummet in paragraph 5 is closest in meaning to

单选题 It can be inferred from paragraph 5 that the author most likely believes which of the following statements about Atlantic eels?

填空题 Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit?
Female yellow eels can grow to more than a meter in length, over twice the length of the males.
A As the elvers grow, they change into adolescent yellow eels, long and slender with brownish-yellow sides and bellies. Yellow eels spend the next several years in fresh water, living in rivers and lakes for up to twenty years. Nearly all of the eels in lakes are female, while those in rivers and estuaries are mainly male. B As the adolescents mature into adults, their eyes grow larger, their backs darken, their sides become silver and their bellies turn silver-white. C They start changing into mature silver eels, ready to begin their southward migration to the Sargasso Sea in late autumn. D The transformation into sexually mature fish occurs deep in the Atlantic, during the long swim back to their mating site.

填空题 An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Atlantic eels are long, slender fish with many fascinating qualities and an epic life journey.

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A. Eels undergo changes in shape and color as they pass through five life phases from larva to adult.
B. The Sargasso Sea is an ideal mating site for eels because of the four surrounding ocean currents.
C. Eels have a very long adolescence and may spend up to twenty years as immature yellow eels in rivers and lakes.
D. Eels hatch in salt water, but spend most of their lives in fresh water before migrating back to the sea to reproduce.
E. Glass eels, yellow eels, and silver eels are captured by different means and for different purposes.
F. Several factors threaten the survival of eels, including altered river systems, overfishing, and climate change.