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问题 选择题

我国黄河流域的黄土高原，在13世纪时曾是肥沃的草原带，而现在不少地区已成为荒山秃岭，土地沙化，其主要原因是[ ]

A．北方寒流长期侵袭

B．地震引起地壳运动

C．人类过度开发，破坏了生态平衡

D．长期干旱，造成沙漠化

答案

答案：C

单项选择题

Why do airplanes take longer to fly west than east

It can take five hours to go west-east from New York(NY) to London but seven hours to travel east-west from London to NY. The reason for the difference is an atmospheric phenomenon known as the jet(喷射) stream. The jet stream is a very high altitude wind which always blows from the west to the east across the Atlantic. The planes moving at a constant air speed thus go faster in the west- east direction when they are moving with the wind than in the opposite direction.

2. What would happen if the gravity on Earth was suddenly turned off

Supposing we could magically turn off gravity. Would buildings and other structures (建筑物) float away What happened would depend on how ply the things were attached to the Earth, The Earth is moving at quite a speed, moving at over a thousand miles per hours. If you turn something around your head on a string(细绳) , it goes around in a circle until you let go of the string. Then it flies off in a straight line. ’ Switching off’ gravity would be like letting go of the string. Things not attached to the Earth would fly off in a straight line. People in buildings would suddenly shoot up-wards at a great speed until they hit the ceiling. Most things outside would fly off into space.

It can be inferred that without gravity（）.

A. buildings and other structures would float away

B. trees and buildings would not easily fly off

C. something around your head would not fly away

D. everything outside buildings would fly off into space
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单项选择题

在下列哪种情况上颌全口他有被动向前的可能？（）

A.平面距上颌牙槽嵴近；

B.左右不平，左高右低；

C.前低后高；

D.平面前高后低；

E.平面平分颌间距离。
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