A、Population. B、Industry. C、Education. D、Welfare. AA是尾句听到的与四个地区有关的原词。符合短文听力“听到什么选什么”的解题原则。

The northern parts of the United States get very cold in the winter. It snows a great deal and the temperature often goes below zero degrees in January, February and March. But the northeastern and north-central regions of the country have been financial and industrial centers, and they are heavily populated. In recent years, people in these regions have begun to take vacations during these cold winter months. They go to southern parts of the country where it is warmer. Many go to Florida where the weather is tropical. Others go to the southwestern states of Arizona, New Mexico and Texas where they find dry desert climates. It has become common, nowadays, for older people to move south to these places when they retire. Typically these people sell their houses in their home communities and move south to begin a new life as senior citizens. Their children likely have homes of their own, and many of them are moving south away from communities where they were raised. The southern and southwestern parts of the country are now growing faster than any other parts. Business and industry have opened many new offices and factories in the south. California is already the most popular state in the country. Texas and Florida are growing fast too. They may soon replace such states as New York and Pennsylvania in population.
19. What does the speaker say about the northern parts of the United States?
20. Why do some retirees sell their houses in their home communities?
21. In which aspect will Texas and Florida surpass New York and Pennsylvania?