Kathy trains nurses. She designed a new assertive communication unit to include in her training. She administers a pretest and then randomly assigns nurses to her treatment or a control condition. She tests their communication skills after the treatment. She sees significant differences at immediate testing but after eight weeks she retests the nurses and finds no differences between her treatment and control students. Which of the following best represents the design of Kathy’s study? Select one:

a. Non-equivalent groups b. Solomon four group c. Pretest-posttest control group d. Post-test only control group