A researcher wanted to determine if using an octane booster would increase gasoline mileage. A random sample of seven cars was selected; the cars were driven for two weeks without the booster and two weeks with the booster. Use the definitions of X, and X, as given in the table. Consequently, D =X - X2. Gasoline Mileage Gasoline Mileage Without booster, X Without booster, X, (mpg) (mpg) 21.2 23.8 25.4 25.6 20.9 22.4 27.6 28.3 22.8 24.5 27.3 28.8 23.4 25.2 What critical value should be used at a = 0.05