听力原文： Last week, you recall, we discussed the early development of railroads in the

United States. Today I want to mention an even earlier form. of transportation, one that brought the first European settlers to America. And thats the wooden-sailing ship. From colonial times sailing ships were vital to the economy. Many coastal towns depended on fishing or whaling for employment and income. This was especially true in the northeastern states. And there the wood from nearby forests and the skills of local designers and workers also formed the basis of an important ship-building industry. But the big profits were to be made on trade with far-away places. And since sea captains often became part owners of their ships, they had a strong interest in the commercial success of their voyages. So these Yankees, thats what US sailors and officers came to be called, they carried on a very profitable trade with other parts of the world. The high point of this trade came in the mid-19th century with the introduction of the clipper ship, the enormous Yankee clippers with huge sails reaching nearly two hundred feet into sky. Hed carry passengers and cargo from New York around South America to San Francisco in less than three months and clear to China in just half a year. At that time this seemed unbelievably fast and efficient. But in the 1860s, more reliable steam-powered ship began to take over. And soon the important role of sailing ships in the U.S. economy would come to an end. Questions 20 to 22 are based on the passage you have just heard. 20. What aspect of United States history does the professor mainly discuss？ 21. According to the professor, what may be one reason for the success of the merchant ships of the United States？ 22. What does the professor say about clipper ships？20．

A．They were protected by a strong United States Navy.

B．They were supported by a well-developed railroad.

C．Most crew members had experience on foreign ships.

D．As part owners of the ships, captains got some of the profits.