You are a policy maker who is concerned with low standardized-test scores in high schools in your school district. You are interested in understanding whether offering special test preparation classes to the students could potentially improve these scores. Towards that end, you randomly select 10 schools and measure the average standardized-test score for these high schools, which turns out to be 69. You randomly select another sample of 10 schools and find that the average score in this sample is 70. As a next step, you offer test preparation classes in the schools in the first group. After 6 months, you find that the score now stands at 78. In the second group, where no test preparation classes are offered, the score after 6 months was recorded at 71. What is the estimated impact of the experiment of offering test preparation classes?a.1b.7c.8d.9