You See a Journal Article Entitled,"Injection of Happystuff Causes a Reduction

You see a journal article entitled,"Injection of Happystuff causes a reduction in symptoms of depression in adult males." This tells you that the independent variable

A) is the injection of Happystuff.
B) is the reduction in symptoms of depression.
C) is the population of adult males.
D) is the construct of depression.
E) cannot be determined.