D

An altimeter(高度计) is an instrument used in an airplane to tell the pilot how high he is flying. The altimeter used in most airplanes is a kind of barometer(气压计), which, like barometer found in ordinary homes, is a measuring instrument for air pressure. The weight of the atmosphere presses downwards everywhere. At sea level this pressure is more than 14 pounds on every square inch of the surface. The higher you go into the air, the lower the air pressure is. An altimeter measures this air pressure to show the altitude of the airplane, or, how high it is above sea level.

But the altimeter does not show how high the plane is above the ground. A plane might be flying at an altitude of 15,000 feet, but it would be only a thousand feet or so above the ground if the land in that area happened to be 14,000 feet above sea level. The pilot adjusts his altimeter to the actual sea-level pressure before leaving the airport and then corrects it in flight by new information given to him by radio. He flies high enough to be above any mountain he may pass on his course .The barometer altimeter is correct within about 300 feet.

Another kind of altimeter, the radio altimeter, makes use of radio reflection. It calculates the height of the flying plane by sending out electrical signals to the surface below and measuring the time required for them to bounce back(反弹). It is correct within 15 feet over water, but is not reliable over land. Big planes usually have both kinds of altimeter.

67. Which of the following diagrams gives the correct relationship between altimeters and barometers? (a----altimeters, b----barometers)

68. The following drawing shows a plane flying over a mountain. Which of the heights given in the drawing is given by the barometer altimeter in the plane at this moment?

69. Suppose a plane using a radio altimeter is flying 10,000 meters above highland which is 4,000 meters above sea level. What reading will be given by the altimeter?

A.10,000meters.   B. 4,000meters.    C.14,000meters.    D. 6,000 meters.

70. Suppose a plane using both kinds of altimeters is flying over a mountainous area where the land rises and falls very abruptly (迅速地). The pilot keeps his plane steady at the same height. What kind of reading will you get on each of the altimeters?

A. The readings on both altimeters will remain steady.

B. The readings on both altimeters will rise and fall abruptly.

C. The readings on the barometer altimeter will rise and fall abruptly, while that on the radio altimeter will remain steady.

D. The readings on the radio altimeter will rise and fall abruptly, while that on the barometer altimeter will remain steady.