City health officials will conduct a two-sample t-test for a difference in means to investigate whether stray dogs in the city have more fleas, on average, than do stray cats. A random sample of 22 stray dogs
had a mean of 120 fleas and a standard deviation of 20 fleas. A random sample of 15 stray cats had a
mean of 100 fleas and a standard deviation of 30 fleas.
Assuming a null hypothesis of no difference, which of the following is a correct test statistic for the test?