A．when they have received satellite picturesB．after the……
A
Now satellites are helping to forecast 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 form. 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 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.
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
