One hundred volunteers who suffer from severe depression are available for a study. fifty are selected at random and are given a new drug, which is thought to be particularly effective in treating severe depression. the other 50 are given an existing drug for treating severe depression. a psychiatrist evaluates the symptoms of all volunteers after four weeks in order to determine if there has been substantial improvement in the severity of the depression. reference: ref 3-10 the factor in this study is a. the extent to which the depression was reduced. b. the use of randomization and the fact that this was a comparative study. c. which drug the volunteers receive. d. the use of a psychiatrist to evaluate the severity of depression.