A consumer-research organization routinely selects several car models each year and evaluates their fuel efficiency. In this year's study of two similar subcompact models from two different automakers were tested. The average gas mileage for 12 cars of brand A was 27.2 miles per gallon, with a standard deviation of 3.8 mpg. The nine brand B cars that were tested averaged 32.1 mpg, with standard deviation of 4.3 mpg. What degrees of freedom must be used to develop the confidence interval for the difference between the two population means

Select one:

A. 16

B. 20

C. 19

D. 21