根据以下材料，回答题
The northern parts of the United States get very cold in the winter.It snows a great deal and thetemperature often goes 21  zero degree in January, 22 and March.But the northeastern andnorthcentral regions of 23 have been financial and industrial centers, and they are heavily polluted.
In recent years, people in these regions have begun to take vacations 24 these cold winter months.They go to southern parts of the country 25 it is wanner.Many go to Florida where the weather is 26  Others go to the southwestern states of Arizona, New Mexico and Texas where they 27 drydesert climates.
It has become 28 nowadays for old people to move south to these places 29 they retire.Typically these people sell their houses in their home communities and move south to begin a new life 30 senior citizens.
Their children likely have homes 31 and many of them are moving south 32 communities wherethey were 33  The southern and southwestern parts of the country are now growing 34 any otherpart.Business and industry 35 many offices and factories in the south.California is already the mostpopular state in the country.
第21题的答案是(　　)