I come from a city on the United States'east coast. It's a ten-hour flight for me to get home from Shanghai. When I got off the plane, I took photos of the blue sky and white clouds along the way. After two weeks, I returned to Shanghai. Much to my surprise, the blue sky in Shanghai that I liked before was not as blue as that in the US. I couldn't help wondering what had happened. A few days later, I went to tour Waitan (a famous tourist attraction in Shanghai) by the Huangpu River with some of my friends. There we saw five Ferrari sports cars by the side of the road. The red and yellow cars looked so nice that everyone was looking at them. However, when I turned my head, I noticed the sky over Waitan, I was shocked to find that the buildings in the Lujiazui area on the other side of Huangpu River could hardly be seen. It was then that I realized that Ferrari cars could be bought with money, but a blue sky couldn't. Shanghai is an important economic   and trade center in China. It has made great contributions   to the whole nation's economic development. However, we need to protect the environment during economic growth. Pollution is poisoning our air and our rivers. Without clean air and rivers, how will people be able to survive? The author mentioned the Ferrari sports cars in order to   _  .
tell us that Shanghai is a developed city ask us to stop driving sports cars tell us how much people like cars focus our attention on air pollution
Question: I come from a city on the United States'east coast. It's a ten-hour flight for me to get home from Shanghai. When I got off the plane, I took photos of the blue sky and white clouds along the way. After two weeks, I returned to Shanghai. Much to my surprise, the blue sky in Shanghai that I liked before was not as blue as that in the US. I couldn't help wondering what had happened. A few days later, I went to tour Waitan (a famous tourist attraction in Shanghai) by the Huangpu River with some of my friends. There we saw five Ferrari sports cars by the side of the road. The red and yellow cars looked so nice that everyone was looking at them. However, when I turned my head, I noticed the sky over Waitan, I was shocked to find that the buildings in the Lujiazui area on the other side of Huangpu River could hardly be seen. It was then that I realized that Ferrari cars could be bought with money, but a blue sky couldn't. Shanghai is an important economic   and trade center in China. It has made great contributions   to the whole nation's economic development. However, we need to protect the environment during economic growth. Pollution is poisoning our air and our rivers. Without clean air and rivers, how will people be able to survive? The author mentioned the Ferrari sports cars in order to   _  .
A. tell us that Shanghai is a developed city
B. ask us to stop driving sports cars
C. tell us how much people like cars
D. focus our attention on air pollution
Answer: