In a Study of the Effectiveness of a Treatment for Depression,a

Multiple Choice

In a study of the effectiveness of a treatment for depression,a psychologist assesses patients' symptoms of depression using a reliable questionnaire both before and for several months after treatment.During the same time period,the psychologist also assesses the symptoms of individuals who are randomly assigned to a waiting-list control group.The results indicate that individuals in both groups experienced a decrease in their symptoms over the 8-month time period of the study.One threat to internal validity the researcher should consider is

A) regression.
B) maturation.
C) selection.
D) all of these