20．
When you look at clouds，you will notice that there are many different kinds．Some clouds are high and thin，some are white and fluffy（蓬松的） and others look dark and heavy．
There are four kinds of clouds，They are high clouds，middle clouds，low clouds，and clouds that grow vertically（垂直的）．And that clouds in these four groups are name for the way they look．
Cirrus clouds are high clouds that are more than 5kilometers above earth．Cirrus clouds look like curly hair，and the word cirrus just means"curl of hair"．Some high clouds that look like layers（層）are called cirrostratus clouds．The word stratus means"layer"．
About 3to 6.5kilometers above earth are the middle clouds．Altostratus clouds are light gray，and Altocumulus clouds are very fluffy．Another cloud that sometimes can be a middle clouds is Nimbostratus cloud．Nimbo comes from the word nimbus，which means"heavy rain"．These clouds make a gray layer from which rain and snow falls．
The low clouds are no higher than about 1.6kilometers above earth．Two kinds of clouds are often found here．Stratocumulus clouds are light and dark．Stratus clouds are very low．
The fourth group of clouds grows vertically．These clouds have a base near the ground but rise to a great height．Cumulus clouds can grow up on top of one another．When they grow up high，they are called cumulonimbus clouds．These clouds can rise as high as 18kilometers．
71．We learn from the passage many names of clouds，and all of them belong toBgroups．
A．two B．four C．six
72．Cirrus clouds and Cirrostratus clouds are given the names by what theyC．
A．are used for B．are made from C．look like
73．It is going to beCwhen we see Nimbostratus clouds in the sky．
A．sunny B．windy C．rainy
74．Clouds that grow vertically rise into the sky in the way of that PictureAshows．
75．Which of the following statements is TRUE？
A．We know the clouds that are more than 3kilometers above earth are called high clouds．
B．Clouds are divided into four groups according to how fat they're away from earth．
C．According to this passage，we know clouds that grow vertically all stay very high in the sky．
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