A、How New York became the largest city in the Unite States. B、How the Eric Canal is built. C、Why New York has attracted so many

M: Are you enjoying this tour?
W: Yes. It’s my first time to visit New York.
M: Mine, too. New York is such a big and fascinating place.
W: Do you know how New York came to be the largest city in the United States?
M: Well, I suppose because it is one of the country’s oldest cities. Besides that, it has a good harbor.
W: That’s true, but there are other old port cities in the U.S. like Boston, Philadelphia, and Baltimore. In fact, in our early history, the first two played a much larger role than New York.
M: The big difference is that New York had a water route to travel to the interior. Other cities lacked that.
W: Do you mean a river?
M: Not a natural one; it was a canal. The Erie Canal, which was completed in 1823, connected New York’s port to the Great Lakes in the Midwest.
W: So even before the railroad was invented, goods and supplies could travel via a water route from New York to the West?
M: Yes, and corn and wheat from that area could travel to New York for sale elsewhere.
W: So New York began to grow faster than the other cities?
M: Yes. In fact, within fifteen years after the canal opened in l823, New York doubled in size. It became the trading capital of the U.S., attracting business, banking and industry, and providing work for thousands of new immigrants.  
Questions 19 to 22 are based on the conversation you have just heard.
19. What’s the main topic of the conversation?
20. Who are the speakers?
21. What gave New York an advantage over the other cities?
22. When was the Erie Canal completed?