Josefina is interested in how a new teaching method will affect math scores of her third-grade students. She allowed her students to choose whether they joined the new method group or remained in the standard teaching group. She administered a standardized math test to her entire class after 3 months. Because she has allowed her groups to self-select, what could be concluded about her study? The study has firmly established a cause-and-effect relationship. The study lacks internal validity. The study does not truly have an experimental or a control group. Josefina can be confident about the statistical significance of her results