To Maximize the Internal Validity of an Experiment, It Is

To maximize the internal validity of an experiment, it is best to make sure that

A) patients are fully aware of which condition of the independent variable they are receiving.
B) a heterogeneous group of patients are selected for the study.
C) the control and experimental groups receive the same experience except for the dependent variable.
D) the control and experimental groups receive the same experience during the study except for the independent variable.