imddung

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 ______.
The answer should be C. ignore the yellow part and pay attention to the green marked one. the clause modifying the "plants" in green is the key for comprehending the question. the real clue of the question is that this herb which has been put under test in lab is a typical example of a group of plants that have the same living conditions which differentiated them from other plants. Therefore, the hypothesis based on the unusual discovery made on the herb will be strengthened if the discovery is a shared character of this special group of plants.   
frankly speaking, we can ignore the word "high", since whether other groups of plants contain H is an information not touched by the statement of the question. therefore, the role of the level of H can't be taken as your reasons. Moreover, in A: "in all parts" doesn't necessarily mean the herb has a high content of H.