Zoologist: Plants preferentially absorb heavy nitrogen from rainwater. Heavy nitrogen consequently becomes concentrated in the tissues of herbivores, and animals that eat meat in turn exhibit even higher concentrations of heavy nitrogen in their bodily tissues. We compared bone samples from European cave bears of the Ice Age with blood samples from present-day bears fed meat-enriched diets, and the levels of heavy nitrogen present in these samples were identical. Thus, the prehistoric European cave bears were not exclusively herbivores. Which one of the following, if true, would most strengthen the zoologist's argument? ['Bone samples from present-day bears fed meat-enriched diets exhibit the same levels of heavy nitrogen as do their blood samples.', 'Plants can also absorb heavy nitrogen from a variety of sources other than rainwater.', 'The level of heavy nitrogen in the bones of any bear fed a meat-enriched diet is the same as that in the bones of any other meat-eating bear.', 'The same number of samples was taken from present-day bears as was taken from Ice Age cave bears.']