A team of psychologists was curious if omega-3 fatty acid supplements could help treat depression in adolescents. They recruited 50 adolescents with varying levels of depression to participate in an experiment. The researchers randomly assigned each subject to take either an omega-3 supplement or a placebo daily, and they monitored subjects' depression levels throughout the study. What is the main reason for randomly assigning the subjects?
A. To identify confounding variables
B. To control against nonresponse
C. To reduce the impact of sampling variability
D. To ensure both the subjects and researchers are blind to who is in which group
E. To create groups that are as similar as possible so the main difference between the groups is the treatment