阅读下列短文,从每题所给的四个选项(A、B、C、D)中,选出最佳选项,并填写在答题卡上 Smog (雾霾) is one of the hot subjects these years. It does great harm to people's health. The most terrible smog lasted for about a week in February, 2014. It covered 15 percent of the land in northern and central China. Now, more and more people in China have realized the importance of protecting the environment. They try to save energy by walking or riding to work in their daily life. Cities including Being and Shanghai have taken action by controlling the number of cars and closing heavily polluting factories. Beijing is heavily polluted, so it's not easy to see blue skies there. But during the APEC summit (经合会议峰会)in November,2014, Beijing saw blue skies for a few days. People happily call that “APEC blue”. “I hope the APEC blue will last, and I believe it will, because China has put great efforts into reducing pollution.” said President Xi Jinping during the summit. The two large countries, China and the USA , produce about 45 percent of the world’s greenhouse gases(温室气体). On November 12, President Xi Jinping and Obama announced that both countries would work together to cut down CO2 emissions (二氧化碳的排放). The goal needs everyone's effort.