The department of motor vehicles wants to check whether drivers have a slower reaction time after drinking two cans of beer. This was done by selecting a sample of 19 drivers and randomly assigning them to one of the 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 is the parameter of interest?
a. The mean reaction times (measured in seconds)
b. The mean difference in reaction times between all those who drink no beers and all those who drink two cans of beer
c. The difference between the mean reaction time of the sample of those who drink no beers and the mean reaction time of the sample of those who drink two cans of beer
d. The difference between the mean reaction time of all those who drink no beers and the mean reaction time of all those who drink two cans of beer