Recent incidents of food contamination have caused great concern among consumers. An article reported that 39 of 80 randomly selected Brand A brand chickens tested positively for either campylobacter or salmonella (or both), the leading bacterial causes of food-borne disease, whereas 64 of 80 Brand B brand chickens tested positive. (a) Does it appear that the true proportion of non-contaminated Brand A chickens differs from that for Brand B