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 toxic to most other plants. A gronomist 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_____.
D. cultivation of the herb in soil with high concentrations of the metals will, over an extened 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 plantapproaches maturity.
The answer is E, but I choose D because of the words bolded. I need your help. | 
