Critical Reasoning

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. Hence, the herb's high histidine production must be the key feature that allows it to grow in metal rich soils.
In evaluating the argument, it would be most important to determine which of the following?

(A) Whether the herb can thrive in soil that does not have high concentrations of the toxic metals
(B) Whether others of the closely related group of plants also produce histidine in large quantities
(C) Whether the herb's high level of histidine production is associated with an unusually low level of production of some other amino acid
(D) Whether growing the herb in soil with high concentrations of the metals will, over time, reduce their concentrations in the soil
(E) Whether the concentration of histidine in the growing herb declines as the plant approaches maturity

[spoiler]Ans (B)[/spoiler]

Why is the answer B and not E?
The conclusion is: "the herb's high histidine production must be the key feature that allows it to grow in metal rich soils."
We are talking about this particular herb, not the others in its group/family. Otherwise the conclusion should have been: "the herb's high histidine production must be the key feature/ helps us to conclude that histidine concentration must be the reason that these group of herbs to grow in metal rich soils."
So why does it matter what happens with the other herbs? If the conclusion was altered, then it makes sense that B should be the correct option.

Why not E since if the herb is still thriving after the histidine has finished, there must be something else keeping it alive, since the authors do not tell us 'for how long' the metals in the soil remain inert...!

I narrowed down to B and E but finally chose E because of the reasoning above.

Noticing details is KEY for getting CR questions right.

Here's the key detail that makes E clearly less right than B.

Conclusion: Hence, the herb's high histidine production must be the key feature that allows it to GROW in metal rich soils.

(E) Whether the concentration of histidine in the growing herb declines as the plant approaches MATURITY

While it may be possible that the plant keeps growing after maturity, I tend to think that the clue for eliminating E is that the use of the expression "approaches maturity" tends to imply that growth is ending.

Now, about B.

The plants are closely related. They all grow thrive in the metal rich soil. One produces histidine.

(B) Whether others of the closely related group of plants also produce histidine in large quantities

Answer this question.

If the answer is "Yes," then likely the histidine is the key feature.

If the answer is "No," then something else is the key feature in the other plants, and given that all the plants are closely related, that feature may be key for the herb too. A possible next step would be to look for another feature.

So B makes more sense than E does. Choose B.

Note, also the importance of modifiers. In B, we're told that the other plant are producing histidine in large quantities. In other words, the histidine level is definitely significant in these other surviving plants, and it's histidine that is supposed to confer benefits that allow the plant to survive.

In E, even if you convince yourself that an eventual decline in histidine production is relevant (I'd argue that it isn't, since the plant is going to die some day after it matures, one way or another) we're told nothing about how significant this decline is. What if histidine production declines by 1%? Would that matter? 5%? 10%? Could the histidine still allow the plant to survive? We don't know.

Thanks Marty and David.

So subtle- Picking up on the different between growth and maturity. I missed that.
Also, the %age decline in histidine explanation was awesome- I could have thought about that a little more, since maybe the decline was only 1%- in case then, E does nothing.

David, could you please explain these two things:
1. "In other words, the histidine level is definitely significant in these other surviving plants, and it's histidine that is supposed to confer benefits that allow the plant to survive."
2, "In E, even if you convince yourself that an eventual decline in histidine production is relevant (I'd argue that it isn't, since the plant is going to die some day after it matures, one way or another)..."

Additionally, I was thrown off also b/c the conclusion reads: herb's high histidine production must be the key feature that allows it to grow in metal rich soils.
If we found out that this particular herb thrives/ grows because of histidine and another closely related plant does too, then it is likely that histidine is the reason.

nk18967 wrote:Thanks Marty and David.

So subtle- Picking up on the different between growth and maturity. I missed that.
Also, the %age decline in histidine explanation was awesome- I could have thought about that a little more, since maybe the decline was only 1%- in case then, E does nothing.

David, could you please explain these two things:
1. "In other words, the histidine level is definitely significant in these other surviving plants, and it's histidine that is supposed to confer benefits that allow the plant to survive."
2, "In E, even if you convince yourself that an eventual decline in histidine production is relevant (I'd argue that it isn't, since the plant is going to die some day after it matures, one way or another)..."

Additionally, I was thrown off also b/c the conclusion reads: herb's high histidine production must be the key feature that allows it to grow in metal rich soils.
If we found out that this particular herb thrives/ grows because of histidine and another closely related plant does too, then it is likely that histidine is the reason.

Happy to. My first point was simply the importance of the word "large." If the other plants produced a tiny amount of histidine, we'd have a hard time arguing that it was the histidine that was allowing the plants to survive. The second point was that we can assume that any plant will go through various phases: growth, maturity, death, and that the way histidine levels change from phase to phase sheds little light on whether histidine is, in fact, the main reason the plant had been able to survive in a difficult environment.