阅读理解。
Now satellites are helping to forecast (预报) the weather. They are in space, and they can reach any
part of the world. The satellites take pictures of the atmosphere (大气), because this is where the weather
forms (形成). They send these pictures to the weather stations. So meteorologists (气象学家) can see the
weather of any part of the world. From the pictures, the scientists can often say how the weather will change.
Today, nearly five hundred weather stations in sixty countries receive satellite pictures. When they receive
new pictures, the meteorologists compare (比较) them with earlier ones. Perhaps they may find that the clouds
have changed during the last few hours. This may mean that the weather on the ground may soon change, too.
In their next weather forecast, the meteorologists can say this.
So the weather satellites are a great help to the meteorologists. Before satellites were invented, the scientists
could forecast the weather for about 24 or 48 hours. Now they can make good forecasts for three or five days.
Soon, perhaps, they may be able to forecast the weather for a week or more ahead (提前).
1. Why do we use the weather satellites to take pictures of the atmosphere? Because _______.
A. the weather satellites can do it easily
B. clouds form there
C. the weather forms there
D. the pictures can forecast the weather
2. Meteorologists forecast the weather _______.
A. when they have received satellite pictures
B. after they have compared new satellite pictures with earlier ones
C. before they received satellite pictures
D. during they study satellite pictures
3. Maybe we'll soon be able to forecast the weather for _______.
A. one day
B. two days
C. five days
D. seven days or even longer
4. The main idea of this passage is that satellites are now used in _______.
A. taking pictures of the atmosphere
B. receiving pictures of the atmosphere
C. doing other work in many ways
D. weather forecasting
从AB边上某点P既垂直于AB边又垂直于磁场的方向射入磁场，然后从BC边上某点Q射出。若从P点射入的该粒子能从Q点射出，则
