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Half of the subjects in an experiment-the experimental group-comsumed 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?
B).A high level of the amino acid in the blood inhibits the synthesis of a substance required for normal brain functioning.
D)the amino acid that is a constituent of the sweetener is also sold separately as a dietary supplement.
为什么答案是B？？？看不懂！！！！
D为什么不对呢？ | 
