This is my question:

The average salary for all 27 Arizona Diamondbacks in the 2005 baseball season was $2,308,488, with a standard deviation of $3,427,508. The population, all National League baseball players in 2005, earned an average salary of $2,585,804, with a standard deviation of $3,500,708. Does the average salary of the Diamondbacks players differ from that of all players in the National League? What statistical analysis would we use to answer this question?

1. single-sample t test
2. z test
3. standard deviation analysis
4. dependent-samplesttest

I need to know why the answer is Z-test? I thought that if the number was under 30 it was categorized under t test, and if it was over 30 it would be a z test?