Ok, allow me to explain this one...
The sentence marked in RED
below is the actual hypothesis.:
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
Now lets go through each option to see if it supports the hypothesis marked in RED
in above argument.
A. histidine is found in all parts of the plant—roots, stem, leaves, and flowers - This does not indicate that the histidine is responsible for the growth of herb in metal-rich soils. So does not support the hypothesis
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 - Same as A. Reject...
C. others of the closely related group of plants are also found to produce histidine in large quantities - Many guys picked up this option. Let me tell you why this is NOT the correct answer. The sentence simply indicates that other closely related group of plants also produce histidine in large quantities. This statement might be true, but it does not prove that histidine is indeed responsible for herb's growth in metal-rich soil. It might be possible that some other element is responsible for the growth of plants, which is not yet being discovered. So this statement does not provide any support to the hypothesis.
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 - huh(?!), REJECT.... I don't think you guys want an explanation for this?
E. the concentration of histidine in the growing herb declines as the plant approaches maturity - BINGO!!!! Read the hypothesis again. It says that the histidine helps the plants to GROW in the metal-rich soil. This is the only option which talks about growth of the herb. If the histidine is only helping the plants to grow, then logically it is not even required once the plant is fully grown. Also, this is the only BEST answer we have. Remember (?) BEST IS NOT IDEAL?
Hope this explanation is helpful.
Consider KUDOS if you found my post helpful
The OA is D. Kindly refrain from posting incorrect OAs. Her's why
Honestly E is not the contender. It has no reference to toxicity