Half of the subjects in an experiment-the experimental group-consumed large quantities of a popular artificial sweetener. Afterward, this group showed lower cognitive abilities than did the other half of the subjects-the control group who did not consume the sweetener. The detrimental effects were attributed to an amino acid that is one of the sweetener' s principal constituents. Which of the following, if true, would best support the conclusion that some ingredient of the sweetener was responsible for the experimental results? ['The two groups of subjects were evenly matched with regard to cognitive abilities prior to the experiment.', 'A second experiment in which subjects consumed large quantities of the sweetener lacked a control group of subjects who were not given the sweetener.', 'The amino acid referred to in the conclusion is a component of all proteins, some of which must be consumed for adequate nutrition.', 'Most consumers of the sweetener do not consume as much of it as the experimental group members did.']