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 help explain how the sweetener might produce the observed effect?
The government's analysis of the artificial sweetener determined that it was sold in relatively pure form.
A high level of the amino acid in the blood inhibits the synthesis of a substance required for normal brain functioning.
Because the sweetener is used primarily as a food additive, adverse reactions to it are rarely noticed by consumers.
The amino acid that is a constituent of the sweetener is also sold separately as a dietary supplement.
Subjects in the experiment did not know whether they were consuming the sweetener or a second, harmless substance.