An Art Therapist Observes Behavioral Changes in George, a Youth

An art therapist observes behavioral changes in George, a youth living in a residential treatment facility. Over the 6 weeks of art therapy treatment, George increases his ability to engage in art-making and participate in group projects. What variable can the art therapist infer has produced the observed effect?

A) Art therapy is more effective for youth in residential treatment than other modalities provided.
B) Art therapy is an effective treatment for youth in residential treatment.
C) Art therapy lowered George's anxiety, allowing him to focus and feel safe enough to work with others.
D) Art therapy was more relaxing than other treatment modalities George was required to participate in.