用所给单词的适当形式填空

     Smog （雾霾）is one of the hot    (subject) these years. It does great harm to (people) health. The (terrible ) smog lasted for about a week in February 2014. It covered 15 percent of the land in northern and ( centre) China. Now more and more people in China have realized the importance of  (protect) the environment , they try to save energy by walking or riding to work in their daily life