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Passage 44 For more than thirty years scientists have been searching for signs of life on other planets. Most of these searches have been done over the radio. The hope is that someone in outer space may be trying to get in touch with us. Scientists also have sent radio and television messages, as well as messages on spaceships traveling through space, on the chance that someone may be receptive to such messages. Scientists are using powerful radio telescopes to "listen" to signals from about 1,000 stars, all within 100 light years of the earth. In addition, they will scan the entire sky to listen for radio messages from more distant stars. Using a computer, they will be able to monitor more than eight million channels at a time. Scientists are looking for any signal that stands out from the background noise. Of the 200 billion stars in the Milky Way galaxy, scientists find that five percent of the stars are like our sun. Perhaps half of them have a planet like the earth. Such a planet would be a reasonable distance from the star for the temperatures to be right for the evolution of life. Believing that there are the inhabitable (that can be lived in) planets in our galaxy, most scientists agree that chances are that one or more of these planets support some life. However, many scientists wonder whether intelligent life exists on other planets. Some believe that thirty years of searching without any intelligent messages show that no one is out there. They say that the evolution of intelligence comparable to ours is unlikely. (44) Other scientists believe that our search hasn’t been long enough to rule out the possibility that intelligent life exists in our galaxy. Although our sun family is only about five billion years old, our galaxy is about 20 billion years old. In that time some scientists think it is likely that civilizations are much more advanced than ours. Perhaps these civilizations send us no signals; perhaps we have not recognized the signals they have sent us. If we hope to find intelligent life, these scientists believe that we have to keep looking. According to the passage, how many planets in our galaxy might be inhabitable
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Passage 39 Many people have difficulty in studying mathematics. Sometimes their difficulty stems from the psychological idea that they are "not good at math". (39) It has been demonstrated that many times that it is often this idea that is causing their problems rather than any lack of skill in mathematics. The best way to deal with this problem is to tell yourself that math involves using the same numbers you have been using all your life and that you know them as well as anyone else. Then set your mind to learning how they are used in the particular part of mathematics you are studying. Many schools offer "math anxiety" courses which are designed to help people overcome their fear of math. Another problem many people encounter is that in mathematics, unlike some other subjects you may have studied, the material has a very important sequence or order. If you do not understand a section of mathematics text, it is no use going on to the next section. Many sections of math books are based on information presented earlier, so it is essential that you understand a section before going on to the next. Sometimes students in mathematics courses do not read the written material in the text but assume that the instructor will tell them what they need to know. This can cause them serious problems, since many instructors assume that students have read the text before coming to class. The written sections of mathematics textbooks contain much valuable information. Sometimes the difficulty of math students comes from______.
Passage 43 Sometimes it is impossible to send all the mail that arrives at the post office. Perhaps there is an inadequate or illegible address and there is no return address. The post office cannot just throw the mail away, so it becomes "dead mail". Dead mail is then sent to one of the U.S. Postal Service’s dead mail offices in Atlanta, New York, Philadelphia, St. Paul, and San Francisco. Seventy-five million pieces of mail can end up in these offices in one year. The staff ’of the dead mail offices has different ways to deal with all of these pieces of dead mail. First of all, they look for clues that can help them deliver the mail; they open packages in the hope that something inside will show where the package came from or is going to. (43) Dead mail will also be listed on a computer so that people can call in and cheek to see if a missing item is there. However, all of the mail cannot simply be stored forever: there is just too much of it. When a lot of dead mail has piled up, the dead mail offices hold public auctions. Every three months, the public is invited to the offices and containing items found in dead mail packages are sold to the highest bidder (出价人). The staff in a dead mail office may open a package in order to find______.
Passage 37 Why don’t birds get lost on their long flights from one place to another Scientists have been puzzled over 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 daylight hours. But what about birds that fly by night (37) 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. The stars are apparently their principal means of navigation. When the stars are hidden by clouds, they seemingly 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. By "his outdoor cousins" the author means______.
Passage 38 Animals do many useful and entertaining jobs. (38) Dogs are particularly valuable in guiding the blind (a trained dog can learn never to cross a busy street when cars are coming, even if his master commands him to do so), protecting property, finding lost people, and hunting criminals. Horses are used in guarding herds, carrying men in lands where there are no roads, and helping farmers plow fields. Pigeons have long been used to carry message. Wild animals from the jungles, forests and seas are very popular performers in circuses and movies. People realize that, although animals may not have the same intelligence as human beings, they are smart enough to learn certain things. The first thing a dog is taught is to obey. It should not take too long for him to learn commands. Simple orders, such as "sit, lie down, stay there, come here" can even be taught by a child. Training a dog to be a watchdog often produces unexpected results. Some dogs quickly learn the difference between unwanted people and friends. This is because their masters welcome friends and invite them into their houses. However, some dogs will always attack the postmen who come to deliver letters. One explanation for this behavior is that, although the postman comes to the house frequently, he never enters the house. Therefore, the dog thinks the postman is someone who is not wanted, but keeps coming back anyway. Masters of dogs who attack postmen can easily show the dogs that the postman is a friend and the dog does not need to treat him as an unwanted person. A dog is quite ready to do what his master wishes. And a dog is always happy when he is praised for proper understanding. A dog feels the happiest if, after he has worked well,______.
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