Read the following research study overview below. Identify all the threats to internal validity and external validity. Give suggestions how this research plan could be improved.

A research plan was conducted to assess if whether a new method to teach history was better than the method currently in use. Sixty 10th grade students (30 in each group) were selected according to their high school GPA - the research wanted the smartest and best students in the study (GPA> 3.5).The students were put in two groups.Group A used the old method (lecture) and Group B used the new method (experiential).Group A members were upset because they weren't chosen for the new method.After the students finished 12th grade (two years later) they were given a history test. Group A students scored an average of 36 points on the test and the remaining 20 students in Group B scored an average of 60 points. A t-test was computed (t=4.68**, p<.0l) and it was determined that the new method was significantly better than the old method. The researcher told the school to adopt the new method of teaching history.