In an effort to improve scores on college-entrance exams, the principal at a large high school chooses 35 students at random who have already taken the exam. The students are enrolled in a six-week prep course to improve their scores. After the prep course, the students take the college-entrance exam again. The mean difference (first – second) in exam scores is –1.75 points with a standard deviation of 7.12 points. Assuming the conditions for inference are met, what is the test statistic for testing the hypotheses ?