At what age do people generally find their first gray hair?

Chemical substances in the nervous system that are similar in structure and in action to opiates—such as heroin and morphine—are ________

a. hormones. b. endorphins. c. androgens. d. neurotransmitters.

________________ America was similar to many current traditional cultures in terms of adolescent work

a. Seventeenth century b. Nineteenth century c. Twentieth century d. Twenty-first century