A、in warm air B、in cold air C、when it reaches the polar region D、only when the clouds are down low A理解题。在Airplanes can’t climb a

Men have 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 airplanes 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, on the other hand, you are not certain to keep clear of had weather even at such heights as 18,000 or 20,000 meters.
    Airplanes 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 airplane 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.