Smokers may have a more difficult time quitting smoking if they live with another smoker. A study comparing bupropion (an antidepressant and smoking cessation aid) with placebo tried to take this into account in their design. The researchers first split the subjects based on whether they lived with another smoker. The subjects who live with another smoker were randomly assigned to take bupropion or a placebo, and those who didn't live with smokers were also randomly assigned to take bupropion or a placebo. The figure shows a flow chart of the design, when 250 of the 429 study subjects lived with nonsmokers and 179 lived with another smoker.

(a) If the two groups of subjects that received bupropion had substantially lower relapse rates than the two placebo groups, can we claim that bupropion is effective as a smoking cessation aid?

(b) Has blocking been used in this study? If so, identify them.

(c) If the placebo group that the subjects lived with another smoker had a higher relapse rate than the placebo group that the subjects did not live with smokers, can we claim that living with smoker(s) makes it harder to quit smoking? Why or why not?