Aleading energy company claims that its top-grade motor oil improves gasoline mileage. an independent testing organization conducts an experiment in which the company's brand (a) is compared to three other competing brands (b, c and d). to conduct the experiment, the testing organization will use these four motor oils in each of three different size automobiles (compact, intermediate, and full size) the automobiles will serve as blocks because of obvious differences in gasoline mileage among them. the data of the experiment consist of the miles per gallon observed for a combination of city and highway travel, as follows: brand size compact 29 26 intermediate full size a 28 • do you see differences in mileage, on average, for these four brands, at 5% level of significance?