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Test One We all know that it is possible for ordinary people to make their homes on the equator(赤道), although often they may feel 1 hot there. Millions do it. But 2 the North Pole ―we know that it is not only a dangerously cold place, but that people like you and me would find it quite 3 to live there. At the present time only the scientists and explorers can do 4, and they use special equipment. Men 5 across and around the equator on wheels. 6 their feet or in ships for thousands of years; but only 7 men, with great difficulty and in very recent time, have ever crossed the ice to the North Pole. So it may 8 you to learn that, when traveling 9, it is really safer to fly over the North Pole 10 over the equator. Of course, this is not true 11 landings in the polar region (which passenger aeroplanes do not make), but the weather, if we are flying at a 12 of 5,000 metres above the Pole, is a delight. At 4,000 metres and more 13 the earth you can always be sure that you will not see a cloud in the sky as 14 the eye can reach. In the tropics(热带), on the other hand, you are not certain to keep15 bad weather even at such heights as 18,000 metres. Aeroplanes cant climb as 16 as or as quickly in cold air as in warm.17 can clouds. In practice, this is an advantage 18 the aeroplane, which is already at a good height 19 it reaches the polar region and so does not need to climb,20 at the same time cold air keeps the clouds down low. | 
