Danny wants to know if students who attend 4-year colleges in U.S. cities earn more upon graduation, on average, than the average college student in the U.S. He assembles a randomly selected, nationwide sample of 100 graduates of urban 4-year colleges and collects data on their annual first year post-graduation earnings. The mean of their earnings is $55,000; the corresponding SD is $12,500. For all 4-year college students in the U.S., the corresponding mean is $45,000. What is value of the relevant test statistic