A recent study showed that the immune system blood cells of the study' s participants who drank tea but no coffee took half as long to respond to germs as did the blood cells of participants who drank coffee but no tea. Thus, drinking tea boosted the participants' immune system defenses. Which one of the following is an assumption on which the argument depends? ['Coffee and tea do not have in common any chemicals that fight disease in the human body.', 'In the study, drinking coffee did not cause the blood cell response time to double.', "Coffee has no health benefits that are as valuable as the boost that tea purportedly gives to the body's immune system.", 'All of the participants in the study drank either tea or coffee, and none drank both.']