科普英语已经成为一个重要的英语文体。它的特点是：1、用词准确，毫不含糊。2、语言结构清楚。3、注重客观叙述，不带感情色彩。
在语法结构上，使用被动语态多，一般现在时态多，句法结构严谨规范，很少有缩略句或不完整句。复合词较多，篇章结构逻辑性强。其实大多数检测阅读理解效果的材料都是科普类文章。只要善于总结它的特点，就会发现，科普英语并不难：主题明确，层次清楚。请你在阅读下面各例的过程中找出它们的主题词。
例1
The family of influenza is among nature's more bizarre concoctions。 Flu viruses of the type known as“A”have shown a capacity to change their molecular structure，stealthily adapting and readapting to their human hosts。These changes happen every year or so，and on a global scale，so that a new strain that appears on one side of the world may spread within months to every continent，to become the new dominant type of influenza。
To those who catch it，it remains the same old illness——fever，aches，sore throat，and cough。 But as far as the body's immunological system is concerned，a new strain represents a new infection，against which antibodies produced by previous strains of influenza are comparatively ineffective，the precise degree depending on how radically the biochemistry of the new strain differs from that of the old。Thus the new strain of influenza can strike a population that is devoid of resistance to produce a raging epidemic。
It is safe to assume that influenza will make a nuisance of itself even during an average winter and in a country with sophisticated medical services，to the extent of causing much discomfort and killing thousands of people，chiefly the old and infirm。 The real anxiety，though，centers on the big global epidemics，or pandemics，which on recent evidence seem to occur about every 10 years。These are produced by a dramatic change in the nature of the virus，a“shift”as opposed to the lesser“drift”that occurs year by year。
关键词提示：influenza，flu virus
例2
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 liberated at the center of the Sun as billions upon billions of nuclei 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 years。
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 wave-length。 This gamma radiation is absorbed by atoms inside the Sun，to be re-emitted at slightly longer wavelengths。This radiation，in its turn，is absorbed and re-emitted。As the energy filters through the layers of the solar interior，it passes through the X-ray part of the spectrum，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。
关键词提示：energy，solar energy
例3
Very high waves are destructive when they strike the land。 Fortunately，this seldom happens。One reason is that out at sea，waves moving in one direction almost always run into waves moving in a different direction。The two sets of waves tend to cancel each other out。Another reason is that water is shallower near the shore。As a wave gets closer to land，the shallow bottom helps reduce its strength。
But the power of waves striking the shore can still be very great。 During a winter gale，waves sometimes strike the shore with the force of 6，000 pounds for each square foot。That means a wave，25 feet high and 500 feet along its face，may strike the shore with a force of 75 million pounds。
Yet the waves，no matter how big or how violent，affect only the surface of the sea。 During the most raging storms，the water a hundred fathoms（600 feet）beneath the surface is just as calm as on a day without a breath of wind。
关键词提示：waves in the sea
例4
Why don't birds get lost on their long flights from one place to another？Scientists have puzzledover this question for many years。 Now they're beginning to fill in the blanks。
Not long ago，experiments showed that birds rely on the sun to guide them during day-light hours。 But what about birds that fly by night？Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights。
A dove had spent its lifetime in a cage and had never flown under a natural sky。 Yet it showed an inborn ability to use the stars for guidance。The bird's cage was placed under an artificial star-filled sky。The bird tried to fly in the same direction as that taken by his outdoor cousins。Any change in the position of the artificial stars caused a change in the direction of his flight。
But the stars are apparently their principal means of navigation。 When the stars are hidden by clouds，they apparently find their way by such landmarks as mountain ranges，coast lines，and river courses。But when it's too dark to see these，the doves circle helplessly，unable to find their way。
关键词提示：flight（migration）of birds
