Washington—For the first time in history, fewer than one American in four lives in the countryside, the US Census (人中普查) Bureau said on Tuesday. Last year’s census found 187.1 million Ameri cans lived in urban are compared with 61.7 million in rural area. America’s 200-year flight from the farm has produced crowded towns. Census number show threefourths of the people live on less than 3 percent of the land. There are more than 2 000 people per square mile (772 per square kilometers) in urban America. In the countryside, on average here are 18 people per square mile (seven per square kilometers). Towns are now crowded because().
选项：

A:more and more people have moved from farms to towns
B:the population is growing fast
C:there are fewer farms now
D:there is only a little land for towns