A、It cost much less. B、It was not affected by weather. C、It was supported by the emperor. D、It was relatively safer. D录音最后提到唐朝之后

The greatest impetus for globalization today is no doubt the internet, but one of the first major drivers of globalization came into being about two thousand years ago. It was the Great Silk Road, which was the oldest, longest, and most historically significant trade route in the world, and it significantly changed the cultures of almost all of continental Eurasia.
    The Silk Road wasn’t actually a single road, though. It was actually a network of trade routes between China and Italy. Travelling across this vast area was difficult, to say the least, and it took centuries for these trade routes to reach completion.
    The Silk Road developed slowly from the East. Its first big impetus came during the Han Dynasty in China, whose emperors reigned from about 200 BC to about 200 AD. China’s warring states had just been united, and the Great Wall of China had just been begun. Since they were still troubled by northern barbarians, the Han emperors extended the Wall westward and sent out emissaries even farther west in search of allies. In 125 BC, one of their generals, Zhang Qian, brought back news of previously unknown peoples in the west, and of a new, large breed of horse that would be invaluable for the Han cavalry. The emperor was very interested, and so more expeditions were sent out. The "heavenly horses", as they were called, were obtained, and Chinese trade missions eventually pushed through to Persia, bringing back many wonderful gifts for the emperor. Zhang Qian is considered by many to be the father of the Great Silk Road.
    Actually, the Silk Road’s name wasn’t coined until the nineteenth century, and silk was never its main commodity, though that fabric must have been very remarkable to Europeans, and it was certainly in demand. The road’s most significant commodity was probably religion—primarily Buddhism, but to a lesser extent, Christianity and Islam as well. Buddhism surged east from northern India in the fourth and fifth centuries AD, where it later reached its height of development in China and Japan.
    The Silk Road’s greatest years of art and civilization came in the seventh century, during the Tang Dynasty. In 754 AD, one of the largest Asian cities, Chang’an, at the eastern end of the road, boasted a population of more than five thousand foreigners from all over Eurasia. After this time, however, the internal stability of China began dissolving, and robbers and neighboring states increasingly pillaged the Silk Road caravans. Eventually, sea trade and sea travel began to replace these slow, unsafe land routes.
22 What do we learn about the Silk Road?
23 What do we learn about Zhang Qian?
24 What is the most significant commodity of the Silk Road?
25 What is the advantage of sea trade and sea travel?