单选题
Please Fasten Your Seatbelts

Severe turbulence (湍流) can kill aircraft passengers. Now, in test flights over the Rocky Mountains, NASA (美国航空航天局) engineers have successfully detected clear-air turbulence up to 10 seconds before an aircraft hits it.
Clear-air turbulence often catches pilots by surprise. Invisible to radar, it is difficult to forecast and can hurl (用力抛出去) passengers about the cabin. In December 1997, one passenger died and a hundred others were injured when unexpected rough air caused a United Airlines flight over the Pacific to drop 300 meters in a few seconds.
However, passengers can avoid serious injury by fastening their seatbelts. "It is the only antidote (对策) for this sort of thing," says Rod Bogue, project manager at NASA"s Dryden Flight Research Center in Edwards, California.
The centre"s new turbulence detector is based on lidar, or laser radar. Laser pulses are sent ahead of the plane and these are then reflected back by particles in the air. The technique depends on the Doppler effect. The wavelength of the light shifts according to the speed at which the particles are approaching. In calm air, the speed equals the plane"s airspeed. But as the particles swirl (打漩) in rough air, their speed of approach increases or decreases rapidly. The rate of change in speed corresponds to the severity (激烈程度) of the turbulence.
In a series of tests that began last month, a research jet flew repeatedly into disturbed air over the mountain ridges (山脉) near Pueblo, Colorado. The lidar detector spotted turbulence between 3 and 8 kilometers ahead, and its forecasts of strength and duration corresponded closely with the turbulence that the plane encountered.
Bogue says that he had "a comfortable amount of time" to fasten his seatbelt. The researchers are planning to improve the lidar"s range with a more powerful beam. The system could be installed on commercial aircraft in the next few years.
单选题 What does " clear-air turbulence " in Paragraph 1 probably mean?
【正确答案】 B
【答案解析】[解析] turbulence意为“湍流”，根据第二段第二句的说法，clear-air turbulence指的是雷达无法测到的一种turbulence，所以选项B为正确答案。
单选题 In December 1997, a United Airlines flight hit unexpected rough air, ______.
【正确答案】 C
【答案解析】[解析] 1997年12月联合航空公司的一架班机在碰到这种气流后，造成一人死亡多人受伤，据此判断，损失惨重，故选C。
单选题 The turbulence detector can tell the severity of the turbulence by measuring ______.
【正确答案】 D
【答案解析】[解析] 第四段最后一句说“The rate of change in speed corresponds to the severity(激烈程度)of the turbulence.”所以选项D为正确答案。
单选题 We can infer from the fifth paragraph that ______.
【正确答案】 A
【答案解析】[解析] 根据第五段的信息，只有选项A符合原文的意思，为正确答案。
单选题 The last paragraph tells us, among other things, that ______.
【正确答案】 A
【答案解析】[解析] 最后一段的第二句“The researchers are planning to improve the lidar"s range with a more powerful beam.”是说，研究人员正计划采用功能更强大的光束来提高雷达的测定范围。据此可以判断，雷达尚需改进，故选A。