America is growing older. Fifty years ago, only 4 out of every 100 people in the United States were 65 or older. Today, 10 out of every 100 Americans are over 65. The aging of the population will affect American society in many ways —education, medicine, and business. Quietly the graying of America has made us a very different society—one in which people have a quite different idea of what kind of behavior is suitable at various ages.

　　A person’s age no longer tells you anything about his/her social position, marriage or health. There，s no longer a particular year in which one goes to school or goes to work or gets married or starts a family. The social clock that kept us on time and told us when to go to school, get a job, or stop working isn’t as strong as it used to be. It doesn’t surprise us to hear of a 29-year-old university president or a 35-year-old grandmother, or a 70-year-old man who has become a father for the first time. Public notions are changing.

　　Many people say, “I am much younger than my mother—or my father—was at my age. No one says “act your age” anymore. We’ve stopped looking with surprise at older people who act in youthful ways.
2. The underlined word “one” refers to _______

A.a society.

B.America.

C.a place.

D.population.