6. Based on the responses she received from over 100,000 solicited letters from GA pi- lots, a representative for the AOPA reports that most GA pilots regret not becoming an ATP. A reasonable evaluation of this finding with respect to the population of GA pilots is that it is

a. representative because of the large sample size.

b. representative because respondents were volunteers.

c. not representative because of the relatively small sample size.

d. not representative because respondents were self-selected.