单项选择题

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 North Pole (北极)-we know that it is not only a dangerously cold place, but that people like you and me 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 had been traveling 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, this is not true about landings in the polar region (which passenger planes do not make), 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 more above the earth you can always be sure that you will not see a cloud in the sky as far as the eye 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.
Planes can’t climb as high or as quickly in cold air as in warm air. Nor can clouds. (18) In practice, this is an advantage to the plane, which is already at a good height when it reaches the polar region and so does not need to climb, while at the same time cold air keeps the clouds down low.
Planes usually do not need to climb quickly or fly high in the polar region because______.