An endogenous opioid is an example of a(n)

Rafael has a cold and has plugged sinuses. These symptoms are most likely to affect

A) his ability to listen to his Art History professor's lecture. B) his enjoyment of the food at the university cafeteria. C) his pain tolerance threshold. D) his sense of balance.

Olivia's mom just found out that for the first time in her life she needs to wear glasses. A likely explanation for this is __________

a) near-sightedness. b) secondary aging. c) menopause. d) presbyopia.