Rats injected with morphine exhibit decreased activity of the immune system, the bodily system that fights off infections. These same rats exhibited heightened blood levels of corticosteroids, chemicals secreted by the adrenal glands. Since corticosteroids can interfere with immune-system activity, scientists hypothesized that the way morphine reduces immune responses in rats is by stimulating the adrenal glands to secrete additional corticosteroids into the bloodstream. Which of the following experiments would yield the most useful results for evaluating the scientists' hypothesis? ["Testing the level of immune-system activity of rats, removing their adrenal glands, and then testing the rats' immune-system activity levels again", 'Injecting rats with corticosteroids and then observing how many of the rats contracted infections', 'Injecting morphine into rats that already have heightened blood levels of corticosteroids and then observing their new blood levels of corticosteroids', "Removing the adrenal glands of rats, injecting the rats with morphine, and then testing the level of the rats' immune-system responses"]