The department of motor vehicles wants to check whether drivers take a longer time to react after drinking two cans of beer. This was done by selecting a sample of 19 drivers and randomly assigning them to one of two groups. The reaction times (measured in seconds) in an obstacle course are measured for a group of 10 drivers who had no beer. The other group of 9 drivers were given two cans of beer each and their reaction times on the same obstacle course are measured. What are the correct degrees of freedom for this test?
A. 7
B. 8
C. 17
D. 18
E. 21