Question: 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 raking 600 mailgrams 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.

(a) What is the name for this experimental design?

(b) Use a diagram to outline the design.

(c) The report of ate study said,

Sixty-eight percent of the runners in the placebo group reported the development of symptoms of upper respiratory tract infection after the race; this was significantly more than that reported by the vitamin C-supplement group( 33%).

Explain to someone who knows no statistics why "significantly more” means there is good reason to think that vitamin C works.