1. A researcher plans to study how participants' attitudes are influenced by the type of celebrity used in commercials. The independent variable is type of celebrity with two levels: music industry and movie industry. Select any possible reasons that the researcher would use a within-group design. a. Because she cannot randomly assign participants to a level of the independent variable. b. To control for other variables, such as age, that may differ between participants and influence the dependent variable. c. Because the researcher has access to a limited number of participants, it is not possible to have more than one group. d. Because the researcher is concerned about order effects, she wants to avoid a between-groups experimental design.