One Hundred Volunteers Who Suffer from Severe Depression Are Available

One hundred volunteers who suffer from severe depression are available for a study.Fifty are selected at random and are given a new drug that 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 4 weeks in order to determine if there has been substantial improvement in the severity of the depression.What is the explanatory variable or factor in this study?

A) Which drug the volunteers receive
B) The use of randomization and the fact that this was a comparative study
C) The extent to which the depression was reduced
D) The use of a psychiatrist to evaluate the severity of depression