一、
　　Dear Bill,
　　I''m glad to receive your letter,thank you for your caring for the weather and my health.Now I''d like to tell you something about the smog.
　　Since the winter last year,the smog has occurred a lot of times.it has done great harm to our daily life.many traffic accidents happened just because of the heavy smog weather,more and more people have to go to see the doctor because the serious disease caused by the smog,quite a lot of flights have to be put off,a