题目内容：
根据下列材料，请回答题Please Fasten Your Seatbelts
Severe turbulence (潮流 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 fore- cast 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.
What does "clear-air turbulence" in Paragraph 1 probably mean? A．A not very rough storm.
B．Unexpected disturbed air.
C．A kind of visible storm.
D．A storm over mountain ridges.
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