A researcher collects data on hydration levels in firefighters responding to an incident. They collect hydration levels on 15 firefighters at the beginning of their shift and then collect hydration levels on the same 15 firefighters at the end of the their shift. If the researcher is trying to determine if hydration levels change during the course of a work shift, the test they should use to analyze the data is called: Select one: O a. independent t-test O b. z-score O c. ANVOA O d. dependent t-test