Chris would like to conduct a study on the effectiveness of a new substance-abuse treatment program.

Chris would like to conduct a study on the effectiveness of a new substance-abuse treatment program. He has a sample of participants go through the program and he uses as a comparison group people who did not volunteer for the program but were eligible. Chris has used a(n) _________ design for his study.
a.nonequivalent group
b.interrupted time-series
c.pretest-posttest control-group
d.Solomon four-group