A soft drink manufacturing and exporting company markets a new brand of a soft drink with two experimental flavors. Flavor 1 is sent to thirteen stores; the average sales in the first month is 45 units with a sample standard deviation of 7 units. Flavor 2 is sent to ten stores; the average sales in the first month is 38 units with a sample standard deviation of 6 units. Assume that the populations from which the samples are taken follow normal distribution.
Calculate the degrees of freedoms for the test.

a. 
22


b. 
23


c. 
9


d. 
12


e. 
21
