A group of psychologists was interested in knowing whether living environment had any effect on a student's grade point average (GPA). They took a set of twins and randomly assigned one sibling of the twins to live in an urban area and the other to live in a rural area After one year, they computed the GPAs for the twins and looked at the differences. The psychologists found the mean difference in the GPAs between the 24 sets of twins was 0.04 with standard deviation 0.25. What is the test statistic