A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants. Agronomists studying the growth of this herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert. Hence, the herb's high histidine production must be the key feature that allows it to grow in metal-rich soils. In evaluating the argument, it would be most important to determine which of the following? ['Whether the herb can thrive in soil that does not have high concentrations of the toxic metals', 'Whether growing the herb in soil with high concentrations of the metals will, over time, reduce their concentrations in the soil', "Whether the herb's high level of histidine production is associated with an unusually low level of production of some other amino acid", 'Whether others of the closely related group of plants also produce histidine in large quantities']