Acloudiswaterdropsorsmalliceinthesky．Therearemanykindsofclouds．CloudsareanimportantpartofEarth'sweather．HowDoCloudsForm？Theskycanbefullofwater．Butmostofthetimeyoucan'tseethewaterneartheground．It

Question

A cloud is water drops or small ice in the sky．There are many kinds of clouds．Clouds are an important part of Earth's weather．
How Do Clouds Form？
The sky can be full of water．But most of the time you can't see the water near the ground．It is usually in the form of water vapor and it is too small to see．When warm air goes higher in the sky，it gets large and cools down．Cool air can't hold as much water vapor as warm air．Some of the vapor gets together in the air and forms a very small drop of water．When lots of these drops come together they become a cloud and then we can see it．
Why Do Clouds Turn Gray？
Clouds are made up of small water drops and small pieces of ice．The water and the ice look white in the sun light．If the clouds get thick enough of high enough all the light above does not make it through，then the clouds look gray or dark．Also，if there are lots of other clouds around，they look gray．
What Are Some Kinds of Clouds？
Clouds get their names in two ways．One way is by where they are in the sky．Some clouds are high up in the sky．Low clouds are near the ground．These clouds are called fog．Middle clouds are between low and high clouds．The other way clouds get their names is by their shapes．
What Makes Rain？
Most of the water in clouds is in very small drops．The drops are so light and they stay in the air．Sometimes those drops meet other drops．Then they turn into bigger drops and then they become too heavy．And finally they come down to Earth．We call the falling water drops"rain"．When the air is colder，the water may form snow．
Why Do People Study Clouds？
Clouds are important．Rain and snow are important for life on Earth．At night，clouds keep the ground warm．In the daytime，clouds can keep us cool．Studying clouds helps people better understand Earth's weather．People use many things to study clouds．

43．From the passage we know that___
A．there are only two kinds of clouds              
B．clouds get their names in three ways
C．warm air can hold as much vapor as cold air    
D．most of the water in clouds is in very small drops
44．People study clouds to___．
A．change Earth's air                    
B．learn what's happening in the sky
C．know what's in the sky             
D．better understand Earth's weather
45．The best title of the passage is___
A．Kinds of Clouds   
B．What are clouds     
C．What makes rain                    
D．How Clouds Help Us．

Answer 1

43．D细节理解题．根据Most of the water in clouds is in very small drops可知在云层中大部分的水是非常小的液滴，故选D．
44．D细节理解题．根据Studying clouds helps people better understand Earth's weather可知研究云有助于人们更好地了解地球的天气．故选D．
45．B主旨大意题．根据A cloud is water drops or small ice in the sky．There are many kinds of clouds．Clouds are an important part of Earth's weather．可知本文介绍了云，故选B．