A two-sample t-test will be conducted to investigate whether the mean number of tickets sold for children each day is less at movie theater J than at movie theater K. From a random sample of 50 days at theater J, the average was 75 children tickets with standard deviation 12. From a random sample of 60 days at theater K, the average was 85 children tickets with standard deviation 14. Under the assumption that there is no difference in the population means (J minus K), which of the following is the appropriate test statistic for the test?