A doctor reported on a study that treated children who had sleep apnea, which interferes with breathing while a child is asleep. In the study, 464 children, 5 to 9 years of age, were randomly assigned to either surgery or to be under constant watch for a certain period of time. The study found that there were significantly greater improvements in behavioral, quality-of-life, and sleep study findings in the group that had surgery than the group assigned to constant watch.

Was the study a controlled experiment or an observational study? Explain how you know.

(A) This is an observational study. The number of treatment groups is not sufficiently large to control for all relevant factors aside from sleep apnea. Having a large number of treatment groups is essential to conducting a controlled experiment.
(B) The study was a controlled experiment. In the study, the researchers controlled for other factors prior to the inclusion of the children to the study. This is essential to conducting a controlled experiment.
(C) The study was an observational study. The control group was under constant watch, which is heavily observational and noninvasive.
(D) The study was a controlled experiment because the children were randomly assigned to either surgery or constant watch. This is essential to conducting a controlled experiment.