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.

E是肯定不对。但D也不太好。Boldface line只是说明它能使这些有毒的金属inert。Inert的意思是：无活动的, 惰性的。这不是化学中的中和---neutralization。所以从D我们只能得出结论：这种植物的分泌物能使有毒的金属不对它有毒害作用。但这种分泌物是不是能使金属的毒中和为无毒而改良了土壤，这里没说。

最好你把其它答案贴出来。。。。

Its quite tricky.

If E is true, when the herb approaches maturity, which also means it stops growing, the production of histidine gets weak

conclusion: histodine decides whether the herb grows or not

The maturity and declining take place at the same time, it would support that histodine is a must.

D saying the herb helps to lower the effect of metals, nothing to do with its growth needed in the conclusion.