Which Statement Is an Example of a Secular Trend

Which statement is an example of a secular trend?

A) African American children mature more quickly than do their European American peers.
B) Females generally reach puberty before their male peers do.
C) The average child growing up in North Korea is several inches shorter than is the average child growing up in North America.
D) Northern American adults are several inches taller than their same-sex great-grandparents were.