阅读短文,从每题所给四个选项（A、B、C和D）中选出最佳选项。

    When Canadian Prime Minister Justin Trudeau visited China in September, where did he take his family? To see the Great Wall, of course!

 Built to keep enemies from the country, the Great Wall is known as one of the eight wonders in the world.

    It was first built in different sections during the Warring States period (战国时期). Later, Emperor Qin Shihuang connected these big walls (sections) together into a very long one that was about 10,000 li (5,000 kilometers). In the Ming Dynasty, people built many new parts of the Great Wall that are still in good condition today.

    Being famous for its wonderful view, the Great Wall draws millions of tourists each year. But now the long wall is in danger. One third of the Great Wall of the Ming Dynasty has disappeared, Xinhua reported. One reason for that is the old age of the Great Wall. Some parts are over 500 years old. Strong wind and rain has slowly destroyed it. Another reason is that people haven't realized they have to protect it. Some people stole the bricks of the Great Wall and sold them for money. And some visitors drew on the Great Wall. On sections of the Wall that many tourists visit, like Badaling, almost every brick is covered with graffiti.

    Our country is making an effort to repair the Great Wall. But it is such a big project that it needs much time to be completed. To make sure that the Great Wall remains a great wonder, every one of us should help protect it ourselves.