Which of the following is the best example of a longitudinal research study?

A) Dr. Ngo studied four groups of children aged six, eight, ten, and twelve to determine how therapy helps children cope with their parents' divorce.
B) To determine how alcohol affects the retention of information, researchers monitored subjects' ingestion of alcohol while the subjects were learning to play a complicated game.
C) Dr. Cisneros initially studied a group of recently-divorced 40-year-olds to evaluate their communication skills, and continued to study their interpersonal relationships for 30 years.
D) In December 2004, researchers surveyed shoppers at local malls to assess peoples' attitudes about retirement and their plans for post-retirement activities.

C