Sociologists Have Adopted the Phrase, "The Graying of the United

Sociologists have adopted the phrase, "the graying of the United States." What does this phrase mean?

A) People are living under so much stress now, that they are going gray early.
B) Gray is culturally considered beautiful and thus people are bleaching their hair.
C) The greatest part of the population is reaching the age of 65 and entering old age.
D) With industrialization and the building of big cities, there is very little green nature left and everything is the color of steel.