Taste buds were the primary tool early humans used for testing foods. Sour taste warns of possible spoilage of food while bitterness is a warning of many poisons. Early humans also recognized sweet foods and salty foods as meeting nutritional needs. So the fact that people can now clearly distinguish these four tastes-sour, bitter, sweet, and salty-is completely explained by people' s use of taste to test for the healthfulness of foods. The reasoning in the argument is most vulnerable to criticism on the grounds that the argument ['takes a necessary condition for the truth of its conclusion to be sufficient to justify that conclusion', 'takes what might be only a partial explanation of a phenomenon to be the complete explanation', 'fails to consider that many people associate foods more with their smells than with their tastes', 'fails to consider that some nutritious foods are bitter when raw but not after being cooked']