A recent study involved feeding a high-salt diet to a rat colony. A few months after the experiment began, standard tests of the rats' blood pressure revealed that about 25 percent of the colony had normal, healthy blood pressure, about 70 percent of the colony had high blood pressure, and 5 percent of the colony had extremely high blood pressure. The conclusion from these results is that high-salt diets are linked to high blood pressure in rats. The answer to which one of the following questions is most relevant to evaluating the conclusion drawn above? ['How many rats in the colony studied had abnormally high blood pressure before the study began?', "How much more salt than is contained in a rat's normal diet was there in the high-salt diet?", 'What percentage of naturally occurring rat colonies feed on high-salt diets?', 'Have other species of rodents been used in experiments of the same kind?']