根据首字母提示完成小短文

The Earth gets s now. In some cities, the air is dirty. There are many cars in these cities. The s from cars makes the air very dirty. So we should s driving more cars. We should walk or ride bikes more often to school or the work place. Walking is a good way to k the air clean. It also makes us h and strong. Let's do more things to make the air clean.