Read the following paragraph and choose the best topic sentence from the four options that follow.Write the corresponding letter of your choice on the answer sheet.
A recent study shows that over 5 million children in the United States suffer from asthma(哮喘).More than3,500 kids in California took part in the study.About half of those kids lived in areas that had lots of air pollution.The other half lived in arcas that had clean air. After studying the kids for five years,doctors found that about 260 of them developed asthma. Most of the kids who developed asthma lived in towns that were heavily polluted.Doctors say that breathing too much polluted air caused the asthma cases.Despite this,it is still recommended that children should play outside but only on days when air pollution isn't serious.