A、In the 1970s. B、In the 1960s. C、In the 1950s. D、In the 1940s. A事实细节题。短文末尾提到，从20世纪50年代起，美国国会拨款建设公路系统，直到1974年，公路系统建设基本完成。

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.
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?