One hundred volunteers who have agoraphobia (fear of open and crowded space) are available for a study. Fifty are selected at random and are given the drug imipramine, which is believed to be effective in treating agoraphobia. The other 50 are given a placebo. A psychiatrist evaluates the symptoms of all volunteers after two months to determine if there has been substantial improvement in the severity of their symptoms. What type of study is this?
A. Matched pair design
B. Observational study
C. Completely randomized block design
D. Completely randomized design