6．听下面一段材料，回答第18至第20题．
18．Why do we use the weather satellites to take pictures of the atmosphere？
A．Because the weather satellites can do it easily．
B．Because clouds form there．
C．Because the weather forms there．
19．When do scientists forecast（预报） 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．
20．How long are they able to forecast the weather nowadays？
A．One day．
B．Three days．
C．Four or seven days．
