Dr Wizard Randomly Assigns People to Two Groups

Multiple Choice

Dr. Wizard randomly assigns people to two groups. Those in the experimental group receive a "subliminal audiotape" that they are told will improve their self-esteem. The other group receives no treatment. Three weeks later Dr. Wizard interviews them and reports that people in the experimental group show higher self-esteem. What change would IMPROVE the procedure for this research?

A) Eliminate the independent variables
B) Use a convenience sample of people
C) Include more demand characteristics
D) Make the procedure double-blind