Identify which threats to internal validity are present in the following study. Then indicate how you can eliminate (reduce) such threats (give an example of how you would do it).

At the beginning of first grade,in one class,a team of teachers measured students' knowledge of the letters of the alphabet. Then the team taughtthe studentsletter recognition using a new instructional computer program. After three months of instruction, the team again measuredthestudents' knowledge of the letters. They found astatisticallysignificant increase in average score from pretest to posttest. Because the difference was larger than what chance alone would create, they concluded that the new program must be the cause of the increased knowledge.