【正确答案】 D

【答案解析】

听力原文：

Passage Two

The United States has one of the best highway systems in the world. Interstate highways connect just about every large and mid-sized city in the country. Did you ever wonder why such a complete system of excellent roads exists? For an answer，you would have to go back to the early 1920s. In those years, just after World War I, the military wanted to build an American highway system for national defense. Such a system could, if necessary, move troops quickly from one area to another. It could also get people out of cities in danger of being bombed. So-called roads of national importance were designated, but they were mostly small country roads. In 1944, Congress passed a bill to upgrade the system, but did not fund the plan right away. In the 1950s, the plan began to become a reality. Over $25 billion was appropriated by congress, and construction began on about 40,000 miles of new roads. The idea was to connect the new system to existing expressways and freeways. And though the system was built mostly to make car travel easier, defense was not forgotten. For instance, highway overpasses had to be high enough to allow trailers carrying military missiles to pass under them. By 1974, this system was mostly completed. A few additional roads would come later. Quick and easy travel between all parts of the country was now possible.

Questions 19 and 21 are based on the passage you have just heard.

19. What does the speaker say about the American highway system?

20. What was the original purpose of building a highway system?

21. When was the interstate highway system mostly completed?

解析：根据文章：The United States has one of the best highway systems in the world. 可以得出答案。