2. The FAA's mandatory retirement age, which was increased from age 60 to age 65 for Part 121 pilots, became effective December 13, 2007. Let's assume that you endeavor to conduct a study to determine if there is any difference in piloting skills between 64-year-old Part 121 pilots and their 60-year-old counterparts, where piloting skills will be measured in a simulator and defined as the mean deviation from the glidepath in degrees. In this study, the group membership variable is and the groups are

a. counterparts, Part 121 pilots

b. 60-and 64-year-old pilots, difference in mean deviation from the glidepath

c. pilot age, 60- and 64-year-old pilots

d. Part 121 pilots, differences in piloting skills