A study of cell phones and the risk of brain cancer looked at a group of 469 people who have brain cancer. The investigators matched each cancer patient with a person of the same sex, age, and race who did not have brain cancer, then asked about use of cell phones. This is
(a) An observational study.
(b) An uncontrolled experiment.
(c) A randomized comparative experiment.
(d) A matched pairs experiment.
(e) A survey.