Which of the following most logically completes the argument?
A certain cultivated heab 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 histidince 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.
不明白答案为什么是E？？请高人指点！！ | 
