The percentage of U.S. adults with a high school education or has:

A. remained constant since the 1960s.
B. risen from 8 percent in 1960 to 31 percent in 2012.
C. risen to over 90 percent in the twenty-first century.
D. risen from 41 percent in 1960 to 88 percent in 2012.

D. risen from 41 percent in 1960 to 88 percent in 2012.