Joel administers an analytical thinking test to a group of charity volunteers in an organization. They are then made to go through an intensive boot-camp on analytical thinking skills. At the end of the boot-camp, the analytical thinking test is given again and a substantial improvement in analytical thinking scores is found. This is an example of a a. nonequivalent control group design b. control series design c. Solomon four-group design d. one-group pretest-posttest design