题目内容
Space is a dangerous place, not only because of meteors (流星) but also because of rays from the sun and other stars. The atmosphere again acts as our protective blanket on earth. Light gets through, and this is essential for plants to make the food which we eat. Heat, too, makes our environment endurable. Various kinds of rays come through the air from outer space, but enormous quantities of radiation from the sun are screened off. As soon as men leave the atmosphere they are exposed to this radiation but their spacesuits or the walls of their spacecraft, if they are inside, do prevent a lot of radiation damage. Radiation is the greatest known danger to explorers in space. The unit of radiation is called "rem". Scientists have reason to think that a man can put up with far more radiation than 0.1 rein without being damaged; the figure of 60 rems has been agreed on. The trouble is that it is extremely difficult to be sure about radiation damage—a person may feel perfectly well, but the cells of his or her sex organs may be damaged, and this will not be discovered until the birth of deformed children or even grandchildren. Missions of the Apollo flights have had to cross belts of high amount of rems. So far, no dangerous amounts of radiation have been reported, but the Apollo missions have been quite short. We simply do not know yet how men are going to get on when they spend weeks and months outside the protection of the atmosphere, working in a space laboratory. Drugs might help to decrease the damage done by radiation, but no really effective ones have been found so far. We know from the passage that ______.
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Where did most immigrants come from between 1820 and 19737 Most immigrants came from .
How many immigrants were permitted to enter the country between 1820 and 1973 immigrants were permitted to enter the country between 1820 and 1973.
There is a new type of small advertisement becoming increasingly common in newspaper classified columns. It is sometimes placed among "situations vacant", although it does not offer anyone a job, and sometimes it appears among "situations wanted", although it is not placed by someone looking for a job, either. What it does is to offer help in applying for a job. "Contact us before writing your application", or "Make use of our long experience in preparing your curriculum vitae or job history" is how it is usually expressed. The growth and apparent success of such a specialized service is, of course, a reflection on the current high levels of unemployment. It is also an indication of the growing importance of the curriculum vitae( or job history), with the suggestion that it may now qualify as an art form in its own right. There was a time when job seekers simply wrote letters of application. "Just put down your name, address, age and whether you have passed any exams," was about the average level of advice offered to young people applying for their first jobs when I left school. The letter was really just for openers, it was explained, and everything else could and should be saved for the interview. And in those days of full employment the technique worked. The letter proved that you could write and were available for work. Your eager face and intelligent replies did the rest. Later, as you moved up the ladder, something slightly more sophisticated was called for. The advice then was to put something in the letter which would distinguish you from the rest. It might be the aggressive approach. "Your search is over. I am the person you are looking for." was a widely used trick that occasionally succeeded. Or it might be some special feature specially designed for the job interview. There is no doubt, however, that it is increasing number of applicants with university education at all points in the process of engaging staff that has led to the greater importance of the curriculum vitae. Nowadays a demand for this specialized type of service has been created because ______.
When did the greatest numbers of immigrants come They came in the years .
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