Fifteen SmartCars were randomly selected and the highway mileage of each was noted The analysis yielded a mean of 47 miles per gallon and a sample standard deviation of 5 miles per gallon Which of the following would represent a 90% confidence interval for the average highway mileage of all SmartCars? a. 47 plusminus (1.753*(5 + 3.8730) b. 47 plusminus (1.345*(5 + 3.8730) c. 47 plusminus (1.765*(5 + 3.8730) d. 47 plusminus (1.645*(5 + 3.8730)