物体是由于______发声的，声音的传播需要______，在15℃空气中，声音的传播速度是______．

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.

The underlined word "shoot" in the 2nd passage probably means（）

A.send for

B.move quickly

C.come out

D.grow quickly