Why is it important in an experiment to test the effectiveness of a drug? a. a placebo makes the sample less representative of the population but the data can still be used to make inferences about the population. b. a placebo is important so that results from subjects given a treatment can be compared with the results from subjects given a placebo. c. a placebo makes the sample more representative of the population. d. a placebo exaggerates any psychological differences between the control group and the treatment group.