I completely agree with you.
A certain cultivated herb is one of a group of closely related plants that thrive in soil withhigh concentrations of metals that are toxic to most other plants. Agronomists studyingthe 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, theherbâ€™s high histidine production is what allows it to grow in metal-rich soils, a hypothesisthat would gain support if ______.
Fact: HerbA thrive in soil with metals.
Fact: A group of closely related plants thrive in soil with metals.
Fact: HerbA produces histidine, which makes metals harmless to plant.
Hypothesis: Histidine is the reason why HerbA thrive in soil with metals.
If we know that those closely related plants that thrive in metal soil all produce histidine, then we could be reasonably sure that histidine is the reason why they thrive in metal soil.
I would choose C.
I don't think we can necessarily get better soils after planting herbA for some time. The passage didn't talk about where the chemical reaction takes place (inside the plant or in the soil), nor did it talk about whether its effect will last so that it actually changes the soil.