[单项选择]

One of the most famous travelers in all of history was Marco Polo. At the age of 17, he left Italy with his father and uncle. It took them more than 3 years to cross the mountains and deserts of Asia. In the year 1275, they reached China. They stayed in China for almost 20 years, as guests of the emperor. He sent them on many trips around his empire. They found China far more advanced than Italy or any other countries in Europe.
After he returned to Italy, Marco Polo dictated many of his stories to a friend, His book, Description of the World, became the most popular book in Europe. People found it difficult to believe his stories of people, animals, places, and things. These are a few of his descriptions:
In one area of China, there were black stones. People dug them out of the mountains. They lit the black stones and they burned very slowly, giving off heat. The people used these stones to cook and to heat their homes.
In China there was a great system o
A. alone.
B. with two friends.
C. with his brothers.
D. with his father and uncle.