新东方背诵50篇41-凯发k8官网

41.american black bears

 

american black bears appear in a variety of colors despite their name. in the eastern part of their range, most of these brown, red, or even yellow coats. to the north, the black bear is actually gray or white in color. even in the same litter, both brown and black furred bears may be born.

 

black bears are the smallest of all american bears, ranging in length from five to six feet, weighing from three hundred to five hundred pounds their eyes and ears are small and their eyesight and hearing are not as good as their sense of smell.

 

like all bears, the black bear is timid, clumsy, and rarely dangerous , but if attacked, most can climb trees and cover ground at great speeds. when angry or frightened, it is a formidable enemy.

 

black bears feed on leaves, herbs. fruit, berries, insects, fish, and even larger animals. one of the most interesting characteristics of bears, including the black bear, is their winter sleep. unlike squirrels, woodchucks, and many other woodland animals, bears do not actually ¹. although the bear does not during the winter ², sustaining itself from body fat, its temperature ³ almost normal, and it breathes regularly four or five times per minute.

 

most black bears live alone, except during mating season. they prefer to live in caves, hollow logs, or ^{4 5}. a little of one to four ⁶ is born in january or february after a ⁷ period of six to nine months, and they remain with their mother until they are ⁸ grown or about one and a half years old. black bears can live as long as thirty years in the wild , and even longer in game preserves set aside for them.

 

 

42.coal-fired power plants

 

the invention of the ⁹ light bulb by thomas a. edison in 1879 created a demand for a cheap, readily available fuel with which to generate large amounts of electric power. coal seemed to fit the bill, and it fueled the earliest power stations. (which were set up at the end of the nineteenth century by edison himself). as more power plants were constructed throughout the country, the reliance on coal increased throughout the country, the reliance on coal increased. since the first world war, coal-fired power plants had a combined in the united states each year. in 1986 such plants had a combined generating capacity of 289,000 megawatts and consumed 83 percent of the nearly 900 million tons of coal mined in the country that year. given the ¹⁰ in the future growth of the nearly 900 million tons of coal mined in the country that year. given the uncertainty in the future growth of nuclear power and in the supply of oil and natural gas, coal-fired power plants could well provide up to 70 percent of the electric power in the united states by the end of the century.

 

yet, in spite of the fact that coal has long been a source of electricity and may remain on for many years(coal represents about 80 percent of united states fossil-fuel reserves), it has actually never been the most desirable fossil fuel for power plants. coal contains less energy per unit of weight than weight than natural gas or oil; it is difficult to transport, and it is associated with a host of environmental issues, among them acid rain. since the late 1960’s problems of ¹¹ control and waste disposal have sharply reduced the appeal of coal-fired power plants. the cost of ameliorating these environment problems along with the rising cost of building a facility as large and complex as a coal-fired power plant, have also made such plants less attractive from a ¹² economic perspective.

 

changes in the ¹³ base of coal-fired power plants could restore their attractiveness, however. whereas some of these changes are intended mainly to increase the productivity of existing plants, completely new technologies for burning coal cleanly are also being developed.

 

 

 43.statistics

 

there were two widely divergent influences on the early development of ¹⁴ methods. statistics had a mother who was ¹⁵ to keeping orderly records of government units (states and statistics come from the same latin root status) and a gentlemanly ¹⁶ father who relied on mathematics to increase his skill at playing the ¹⁷ in games of chance. the influence of the mother on the offspring, statistics, is represented by counting, measuring, describing, ¹⁸, ordering, and the taking of censuses—all of which led to modern descriptive statistics. from the influence of the father came modern inferential statistics, which is based squarely on theories of probability.

 

describing collections involves tabulating, ¹⁹ and describing collections of data. these data may be ²⁰ such as measures of height, intelligence or grade level------variables that are characterized by an ²¹ continuum---or the data may represent ²² variables, such as sex, college major or personality type. large masses of data must generally undergo a process of summarization or reduction before they are comprehensible. descriptive statistics is a tool for describing or summarizing or reducing to comprehensible form the properties of an otherwise unwieldy mass of data.

 

inferential statistics is a formalized body of methods for solving another class of problems that present great of problems characteristically involves attempts to make predictions using a sample of observations. for example, a school ²³ wishes to determine the proportion of children in a large school system who come to school without breakfast, have been ²⁴ for flu, or whatever. having a little knowledge of statistics, the superintendent would know that it is unnecessary and ²⁵ to question each child: the proportion for the sample of as few as 100 children. thus , the purpose of inferential statistics is to predict or estimate characteristics of a population from a knowledge of the characteristics of only a sample of the population.

 

 

44.obtaining fresh water from ²⁶   

 

the concept of obtaining fresh water from icebergs that are towed to populated areas and ²⁷ regions of the world was once treated as a joke more appropriate to cartoons than real life. but now it is being considered quite seriously by many nations, especially since scientists have warned that the human race will ²⁸ its fresh water supply faster than it runs out of food.

 

²⁹ are a possible source of fresh water that has been overlooked until recently. three-quarters of the earth’s fresh water supply is still tied up in glacial ice, a reservoir of untapped fresh water so immense that it could sustain all the rivers of the world for 1,000 years. floating on the oceans every year are 7,659 trillion metric tons of ice encased in 10000 icebergs that break away from the polar ice caps, more than ninety percent of them from antarctica.

 

huge glaciers that stretch over the shallow ³⁰ shelf give birth to icebergs throughout the year. icebergs are not like sea ice, which is formed when the sea itself freezes, rather, they are formed ³¹ on land, breaking off when glaciers spread over the sea. as they drift away from the polar region, icebergs sometimes move mysteriously in a direction opposite to the wind, pulled by subsurface currents. because they melt more slowly than smaller pieces of ice, icebergs have been known to drift as far north as 35 degrees south of the equator in the atlantic ocean. to corral them and ³² them to parts of the world where they are needed would not be too difficult.

 

the difficulty arises in other technical matters, such as the prevention of rapid melting in warmer climates and the ³³ of fresh water to shore in great volume. but even if the icebergs lost half of their volume in towing, the water they could provide would be far cheaper than that produced by desalinization, or removing salt from water.

 

 

45.the source of energy

 

a summary of the physical and chemical nature of life must begin, not on the earth, but in the sun; in fact, at the sun’s very center. it is here that is to be found the source of the energy that the sun constantly pours out into space as light and heat. this energy is librated at the center of the sun as billions upon billions of ³⁴ of hydrogen atoms collide with each other and fuse together to form nuclei of helium, and in doing so, release some of the energy that is stored in the nuclei of atoms. the output of light and heat of the sun requires that some 600 million tons of hydrogen be converted into helium in the sun every second. this the sun has been doing for several thousands of millions of year.

 

the nuclear energy is released at the sun’s center as high-energy gamma radiation, a form of electromagnetic radiation like light and radio waves, only of very much shorter ³⁵. this gamma radiation is absorbed by atoms inside the sun to be reemitted at slightly longer ³⁶. this radiation , in its turn is absorbed and reemitted. as the energy filters through the layers of the solar interior, it passes through the x-ray part of the ³⁷ eventually becoming light. at this stage, it has reached what we call the solar surface, and can escape into space without being absorbed further by solar atoms. a very small fraction of the sun’s light and heat is emitted in such directions that after passing unhindered through interplanetary space, it hits the earth.

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