Which of the following most logically completes the argument?
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 herb have discovered that it produces large amounts of histidine, an amino acid that,
in test-tube solutions, renders these metals chemically inert. Possibly, therefore, the
herb's high histidine production is what allows it to grow in metal-rich soils, a hypothesis
that would gain support if ______.
A. histidine is found in all parts of the plant-roots, stem, leaves, and flowers
B. the herb's high level of histidine production is found to be associated with an
unusually low level of production of other amino acids
C. others of the closely related group of plants are also found to produce histidine in
large quantities
D. cultivation of the herb in soil with high concentrations of the metals will, over an
extended period, make the soil suitable for plants to which the metals are toxic
E. the concentration of histidine in the growing herb declines as the plant approaches
maturity

guys, please lemme know what is the answer to this question.. Oa given is E but i doubt it.. IMO D