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问题 选择题

下列关于无线电广播需要对电磁波进行调制的原因，正确说法的是（   ）

A．把低频信号变成高频信号

B．经过调制后的电磁波在空间传播波长才能不变

C．经过调制后的电磁波在空间才能传播得快

D．经过调制后的高频电磁波才能把我们要告知对方的讯号传递出去

答案

答案：D

调制是把低频信号加到高频电磁波上增强发射能力，解调后两者再分离，故A错.电磁波的波长、频率、波速在真空中不变.

单项选择题

下列有褶皱的叙述不正确的是（）

A、褶皱构造中岩层的连续性未受到破坏

B、褶皱是岩层产生的一系列波状弯曲

C、褶皱核部为新地层，翼部为老地层

D、褶皱的基本类型有背斜和向斜
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单项选择题

A volcanic eruption in Iceland has sent ash across northern Europe. Airlines have stopped or changed the flights across the Atlantic Ocean, leaving hundreds of passengers stuck in airports.
Grimsvotn is one of the largest active volcanoes in Europe. What makes Grimsvotn different is that it lies under a huge glacier(冰川) of ice up to 12 meters thick. The hot volcano heats up the ice above it, which then forms a layer(M) of water between the glacier and the volcano. This layer of water puts pressure on the volcano ,keeping it stable. As the water flows out from under the glacier, the pressure Rifts. The lava(岩浆) from the volcano then comes up to the surface. This is exactly what has happened today.
Now, airlines have to make changes to their flights so as not to fly through the clouds of volcanic ash. According to KLM, one of Europe’s biggest airlines, airplanes cannot go under the cloud or over it. Going through the cloud can result in ash getting stuck in the airplane’s engines, causing damage to the plane.
The eruption has also caused problems for animals in Iceland. The volcano left ash and sharp, glass-like rocks all over the countryside. Farmers are keeping their animals inside to stop them from eating ash-covered grass or the sharp objects.

What makes Grimsvotn different from other volcanoes

A．It is below ice.

B．It lies under the sea.

C．It is the largest volcano.

D．Its lava affects the airlines.
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