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For more than a decade, surgeons have been using a laser procedure to drill holes in the heart as a treatment for chest pain. Many patients report a lasting dramatic decrease in angina (chest pain) symptoms. Is the relief due to the procedure, or is it a placebo effect? Researchers at Lenox Hill Hospital in New York City conducted a completely randomized experiment to investigate this question. A group of 298 volunteers with severe chest pain were randomly assigned to get the laser or not. Both groups had a catheter placed into their hearts. For patients in the laser treatment group, the laser was applied through the catheter. The patients were sedated but awake. The doctors discussed the laser process in the operating room so that each patient thought that he or she was receiving the treatment. a) What are the explanatory and response variables? b) How did the experiment design control for confounding variables? c) The researchers were surprised to discover that the placebo group showed more improvement in pain relief than the laser group! How could this be