Researchers at the University of Wisconsin at Madison conducted an experiment to study the effect of meditation on the immune system. The researchers randomly assigned volunteers to one of two treatments. One group completed an 8-week course in meditation and actively practiced meditation during this period. The other group received no meditation training and did not meditate during the 8-week period. At the end of the experiment researchers gave the subjects a flu shot. A healthy immune system will produce antibodies in response to a vaccine. At 4-weeks and 8-weeks after the flu shot, researchers did a blood test to measure the levels of antibodies produced against the vaccine. Both groups had an increased level of antibodies, but the meditators had a significantly greater increase in antibodies than the non-meditators. Psychosomatic Medicinepublished the results in 2003 in an article titled "Alterations in Brain and Immune Function Produced by Mindfulness Meditation." Stress can harm the immune system. So stress could be a confounding factor in this experiment. Which of the following features of the experiment design is the best way to control the effects of stress on the results of the experiment?

a. use volunteers (bople who have time to volunteer for a study are probably not stressed out by a busy life)
b. use random assignment to treatment groups
c. use a control group for comparison