Researchers Are Attempting to Assess If Cognitive Behavioural Therapy (CBT)is

Researchers are attempting to assess if cognitive behavioural therapy (CBT) is an effective treatment for depression.Participants with unipolar depression are assessed and then randomly assigned to either a treatment group or a control group.The treatment group receives CBT twice per week for 6 weeks.The control group receives no treatment.At the end of the 6 weeks,the depression levels of both groups are reassessed.This study found that CBT treatment was more effective than no treatment but is limited because it did not control for:

A) natural improvement.
B) placebo effects.
C) preexisting differences among participants.
D) participant gender.