An ultramarathon, as you might guess, is a footrace longer than the 26.2 miles of a marathon. Runners commonly develop respiratory infections after an ultramarathon. Will taking 600 milligrams of vitamin C daily reduce these infections? Researchers randomly assigned ultramarathon runners to receive either vitamin C or a placebo. Separately, they also randomly assigned these treatments to a group of nonrunners the same age as the runners. All subjects were watched for 14 days after the big race to see if infections developed. (E. M. Peters et al., "Vitamin C supplementation reduces the incidence of postrace symptoms of upper-respiratory tract infection in ultramarathon runners," American Journal of Clinical Nutrition, 57 (1993), pp. 170-174.) (a) What type of study is this? Observational study: prospective Observational study: retrospective Observational study: cross-sectional Experiment: matched pairs design Experiment: randomized block design Experiment: completely randomized design (b) What are the explanatory variables? Select all that apply. Gender (male, female) Vitamin C supplementation (yes, no) Respiratory infection (yes, no) Runner (yes, no) (c) What is the response variable? Gender (male, female) Respiratory infection (yes, no) Runner (yes, no) Vitamin C supplementation (yes, no)