单项选择题
| We all know that it is possible for
ordinary people to make their homes on the equator (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
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. (76) 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. (77) 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. |