A、They carried passengersbut not cargo. B、They were large, but surprisingly fast. C、They were the first successful steam-powered

Listen to a talk in a class about United States history.
    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 that’s the wooden sailing ship. From colonial times sailing ships were vital to the economy. My 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 shipbuilding 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, that’s what US sailor 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 the sky. He’d 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 unbelievable 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 US economy would come to an end. Now listen to the following recording and answer questions 19 to 21.
19. What aspect of United States history does the professor mainly discuss?
20. According to the professor, what may be one reason for the success of the merchant ships of the United States?
21. What does the professor say about clipper ships?