A researcher did an experiment in which he had one group of people drink a liter of coffee in an hour and a control group of people drink a liter of water within the same amount of time. He then measured their urine output over the course of the next two hours. Both groups produced an average of 250 ml of urine per hour over that time period. He concluded that the coffee itself had no effect on urine output. Given his results, what else should he measure to identify the effects of coffee drinking? He should also measure the concentration of caffeine in the coffee that the test subjects drink to find a correlation between urine output and caffeine. He should also measure the size of the kidneys to see if any water not excreted as urine by his test subjects is being stored by the kidneys. He should also measure the amount of water vapor exhaled by the test subjects to capture any water lost through the lungs. He should also measure the amount of sweat and water in bowel movements produced by the test subjects to see if coffee contributes to water loss through other excretory channels.