阅读材料，回答题。 We all know that it is possible for ordinary people to make their homes on the equator (赤道), although often they may feel uncomfortably hot there. Millions do it. But as for the NorthPole (北极)we know that it is not only a dangerously cold place, but that people like you and I would find it quite impossible to live there. At the present time only the scientists and explorers can do so, and they use special equipment. Men have been travelling across and around the equator on wheels, on their feet or in ships for thousands of years; but only a few men, with great difficulty and in very recent time, have ever crossed the ice to the North Pole. So it may surprise you to learn that, when traveling by air, it is really safer to fly over the North Pole than over the equator. Of course, it is not true about landings in the polar region, but the weather, if we are flying at a height of 5,000 meters above the Pole is a delight. At 4,000 meters and move above the earth you can always be sure that you will not see a cloud in the sky as far as your eyes can reach. In the tropics ( 热带), on the other hand, you are not certain to keep clear of bad weather even at such heights as 18,000 or 20,000 meters. Aeroplanes can’t climb as high or as quickly in cold air as in warm. Nor can clouds. In practice, this is an advantage to the aeroplane, which is already at a good height when it reaches the polar region and so does not to climb, while at the same time cold air keeps the clouds down low. "Millions do it. " in this sentence "do it" refers to ______ 查看材料