#10 - Rhizobium bacteria living in the roots of bean plants

[Administrator]

Complete Question Explanation

Assumption. The correct answer choice is (E)

This stimulus may seem tricky because of the scientific language, but the argument is fairly simple. Rhizo (for short) produces fixed nitrogen in beans and other legumes—reducing the need for fertilizer. The author then concludes that if Rhizo can be put into wheat, it will reduce the need for wheat fertilizer.

This is a Supporter Assumption question, so we should look for the rogue elements that must be linked by the correct answer choice. Where is the logical leap here? The author presumes that Rhizo will do the same things for wheat that it will do for beans. The correct answer choice must link wheat with fixed nitrogen. Correct answer choice (E) is the only one which does so.

[scyq6@sina.com]

Why is answer choice B wrong?

Only when fixed nitrogen is the only soil nutrient that artificial fertilizer provides for wheat crops, the R bacteria can be a perfect substitute. If there are other elements that are needed by the wheat crops, the R bacteria cannot be sufficient for their growth.

Am I thinking too much?

[Clay Cooper]

Hi scyq6,

I don't think you are thinking too much, I think you just lost sight of what the conclusion is.

The conclusion is: if we can create wheat strains that host R bacteria in their roots, the need for artificial fertilizer will be reduced.

Must B be true for this conclusion to be true? No - even if B were wrong (even if fixed nitrogen is not the only thing wheat needs to grow), we could still reduce the need for artificial fertilizer by creating wheat that has R in its roots and thus makes its own fixed nitrogen, because wheat still does need fixed nitrogen.

That is how we use the negation technique to eliminate B: we negate B to produce the following claim:

Fixed nitrogen is not the only thing wheat needs to grow.

and then we ask ourselves if that claim is an attack on the conclusion. If it is an attack on the conclusion, then B is a strong answer choice. If it is not an attack on the conclusion (as here), then B is not a strong answer choice, because this technique has revealed to us that B doesn't have to be true for the conclusion to be true.

I hope that helps! Trust the negation technique - it's a little foreign and hard to use at first but it will never let you down.

[biskam]

I want to make sure that my application of the Assumption Negation technique to B is correct...

There are other nutrients (let's say calcium) that must be supplied by artificial fertilizer for growing wheat crops.

If this is true, it doesn't weaken the conclusion that if biotech can produce weed hospitable to Rhizo, the need for artificial fertilizers will be reduced.

Because (this is the part that I need assurance on) the fertilizers will only be reduced, not eliminated, so wheat can continue to be fed its other necessary nutrients, like calcium, through fertilizers.

Follow up q: is the conclusion the entirety of what i stated, or is it merely "the need for artificial fertilizers will be reduced," thus making the biotech+rhizo+wheat part a premise?

Thank you!

[Eric Ockert]

Hey there!

Yes, your conclusion is spot on. It is the entire conditional rule in the last sentence.

And your analysis is correct too. Since answer choice (B) says that fixed nitrogen is the only soil nutrient that must be supplied by artificial fertilizer, the negation of that answer would be that fixed nitrogen is not the only one. This would not weaken the argument that fertilizer would need to be reduced, only a similar argument that it would need to be eliminated. That's really the trick to B, it's pretty much a Shell Game Answer. While it would be an assumption of a very similar argument to this one, it is not an assumption of the argument as written.

Hope that helps!

[lsacgals101]

Complete Question Explanation

Assumption. The correct answer choice is (E)

This stimulus may seem tricky because of the scientific language, but the argument is fairly simple. Rhizo (for short) produces fixed nitrogen in beans and other legumes—reducing the need for fertilizer. The author then concludes that if Rhizo can be put into wheat, it will reduce the need for wheat fertilizer.

This is a Supporter Assumption question, so we should look for the rogue elements that must be linked by the correct answer choice. Where is the logical leap here? The author presumes that Rhizo will do the same things for wheat that it will do for beans. The correct answer choice must link wheat with fixed nitrogen. Correct answer choice (E) is the only one which does so.

Hi,

I am wondering how you knew this was a supporter assumption question before knowing what the answer was. Is there some kind of indication in the stimulus that we might use to determine whether its a supporter or defender assumption question before proceeding to the answer?

Thanks!

[lsacgals101]

Also, I am having trouble understanding why answer C is wrong.

[Jeremy Press]

Hi lsacgals101,

You can definitely get hints from the stimulus whether a question is likely to be a Supporter or Defender Assumption.

One of the strongest stimulus hints of a Supporter Assumption (though it's not present in this stimulus) is the presence in the conclusion of a "new" idea or concept (crucial to the conclusion) that has not been mentioned in the premises. If you see the conclusion of the stimulus suddenly refer to something you haven't seen mentioned in the premises, one of the assumptions (and the one most likely to be tested) is that something fills the gap between premises and conclusion. In other words, some link to the premises supports that new information in the conclusion.

One of the strongest stimulus hints of a Defender Assumption is some obvious oversight or error in reasoning that would open the argument up to invalidation: e.g., if the argument reaches a definitive causal conclusion, that argument likely has not ruled out all possible alternate causes. So one assumption would be that those alternate causes are not possibilities (thus defending the argument from attack).

In this case, the Supporter nature of the stimulus is a little more subtle, but it has to do with the fact that the argument hasn't fully supported the notion that Rhizobium bacteria in wheat will do what wheat needs done to reduce the use of artificial fertilizers: i.e., supply fixed nitrogen. Without that link (the supply of fixed nitrogen provided to wheat roots by Rhizobium), we cannot have any confidence that Rhizobium would allow a reduction in the need for artificial fertilizers that currently supply it. One small hint that fixed nitrogen is an important component that has to be carried into the conclusion is that the premises say it "must" be supplied (in wheat, that happens normally by fertilizer).

Answer choice C is incorrect because it's not truly necessary to the conclusion. Imagine the logical opposite of answer choice C were true: that "there are SOME [i.e. "at least one"] naturally occurring strains of wheat or other grasses that have Rhizobium bacteria living in their roots." Even if there were at least one such naturally occurring strain, nevertheless biotechnology producing MORE strains could still reduce the need for artificial fertilizers. In other words, the biotechnology might still make those strains more accessible and reduce the fertilizer need. Since the negated form of answer choice C does not definitively destroy the conclusion, it is not truly necessary.

I hope this helps!

Jeremy