[单项选择]At one time or another we have all stopped to think about the weather. Some days the weather is good. Some days the weather is bad. Still, there are days when the weather seems to change from hour to hour. A day that starts out fine might not stay that way. Sometimes a day that seems bad turns out better than you thought it would. There are ways of understanding the weather. One way is to study the clouds.
No two clouds are ever alike. But it is still possible to group clouds and to give them names. We can recognize the different kinds of clouds and watch them for signs of changing weather.
As a rote, the higher the clouds, the better the weather. And the lower the clouds, the worse the weather is likely to be. To predict the weather, there are three things about clouds you should look for: movement, color, and change.
Clouds can tell you if storm is on the way. For example, small feathery cirrus clouds (卷云) can become thick and move lower. This means rain is on the way.
A. learn about the change of weather
B. group them and give them names
C. tell the differences in clouds
D. find the likeness of clouds

[单项选择]At one time or another we have all stopped to think about the weather. Some days the weather is good. Some days the weather is bad. Still, there are days when the weather seems to change from hour to hour. A day that starts out fine might not stay that way. Sometimes a day that seems bad turns out better than you thought it would. There are ways of understanding the weather. One way is to study the clouds.
No two clouds are ever alike. But it is still possible to group clouds and to give them names. We can recognize the different kinds of clouds and watch them for signs of changing weather.
As a rote, the higher the clouds, the better the weather. And the lower the clouds, the worse the weather is likely to be. To predict the weather, there are three things about clouds you should look for: movement, color, and change.
Clouds can tell you if storm is on the way. For example, small feathery cirrus clouds (卷云) can become thick and move lower. This means rain is on the way.
A. the weather always remains the same in a day B) a fine day can’t last for long
B. we rarely think about the weather
C. the weather changes a lot

[单项选择] We all have offensive breath at one time or another. In most cases, offensive breath emanates from bacteria in the mouth, although there are other, more surprising causes. Until a few years ago, the most doctors could do was to counsel patients with bad breath about oral cleanliness. Now they are finding new ways to treat the usually curable condition. Bad breath can happen whenever the normal flow of saliva slows. Our mouths are full of bacteria feeding on protein in bits of food and shed tissue. The bacteria emit evil-smelling gases, the worst of which is hydrogen sulfide. Mouth bacteria thrive in airless conditions. Oxygen-rich saliva keeps their numbers down. When we sleep, for example, the saliva stream slows, and sulfide-producing bacteria gain the upper hand, producing classic " morning breath". Alcohol, hunger, too much talking , breathing through the mouth during exercise — anything that dries the mouth produces bad breath. So can stress, though it’’s n
A. thrive on
B. originate from
C. account for
D. descend from