Explanatory variable and response variable. Identify the explanatory variable and response variable in each of the studies described here. (a) A study of cell phone use and primary brain cancer suggested that cell phone use was not associated with an elevated risk of brain cancer.k (b) Records of more than three-quarters of a million surgical procedures conducted at 34 different hospitals were monitored for anesthetics safety. The study found a mortality rate of 3.4% for one particular anesthetic. No other major anesthetics was associated with mortality greater than 1.9%.' (c) In a landmark study involving more than three-quarters of a million individuals in the United States, Canada, and Finland, subjects were randomly given either the Salk polio vaccine or a saline (placebo) injection. The vaccinated group experienced a polio rate of 28 per 100,000 while the placebo group had a rate of 69 per 100,000. A third group that refused to participate had a polio rate of 46 per 100,000.m