Kaplan Learning Center offers a GRE prep class. To gauge its effectiveness, Kaplan takes a random sample of 85 people enrolled in the prep class. The average GRE score for each student in the sample...

Kaplan Learning Center offers a GRE prep class. To gauge its effectiveness, Kaplan takes a random sample of 85 people enrolled in the prep class. The average GRE score for each student in the sample before enrolling in the prep class is compared to the average GRE score taken upon completion of the course. Which test of significance should be used to determine if there is a statistically significant difference? Why? Should it be one or two tailed?