Healthy lungs produce a natural antibiotic that protects them from

C it is.

People with cystic fibrosis has an abnormally high salt concentration, and high salt concentration causes they cannot to fight off such bacteria.

Ask for support.

No high salt concentration makes people with cystic fibrosis be able to fight off such bacteria.

An typical question.

If A, then B. If not A, then not B.

I am torn between B and C. They seem to be both equally likely

Note also the use of 'unusally low salt cocentration' in (B). How low is low, we do not know.

Agree with rsaraiya; C it is.

The conclusion is : high salt concentration is what makes the antibiotic ineffective to people with cystic fibresis

According to the argument :
Healthy lungs produce a natural antibiotic that protects them from infection
People with cystic fibrosis are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic
The fluid on airway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration
Now, let see the answer choices one by one :

A. When the salt concentration of the fluid on the airway surfaces of healthy people is raised artificially, the salt concentration soon returns to normal.
OUT OF SCOPE : We are concerned with people with cystic fibrosis not with people with healthy lungs.
B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
doesn't mention people with cystic fibresis, hence this option does'nt strenghten the argument
C. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.
Correct , this explains well why the antibiotic is ineffective while the salt concentration is HIGH because when normal , the antibiotic is known to be effective
D. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.
the argument never mentioned synthetic antibiotic though it cannot be the correct answer
E. High salt concentrations have an antibiotic effect in many circumstances.
Clearly Irrelevant

GMATNinja VeritasPrepKarishma GMATNinjaTwo

Can you please explain B / C?
Why do none of answer choices talk about high salt concentration?

I need to strengthen scientist's hypothesis which is: high salt concentration is one which makes the antibiotic ineffective.
What evidence I have?


Some abnormality called CF makes people less immune in spite of sufficient quantity of antibiotics produced


The fluid in lungs of people with CF have very high salt concentration.

So author concludes that high salt concentration makes antibiotic ineffective and people less immune to bacteria.

Is below reasoning correct?
Cause: high salt concentration
Effect: anitibiotics are made ineffective

One way to strengthen A leads to B is:
No A -> no B

I think this is what (C) does.

For (B) can I assume it as same as premise since we are mentioned about :
high concentration makes antibiotic ineffective in argument
B says:
low concentration makes antibiotic effective in argument

Is not logical opposite of:
high concentration : normal concentration and low concentration?

adkikani, your explanation of (C) sounds good! If we take away the cause (high salt concentration), the effect goes away. This suggests that high salt concentration was indeed the cause.

All (B) tells us is that an unusually low concentration of salt is NOT a problem. This does not necessarily suggest that an unusually high concentration would be a problem. What if salt concentration is irrelevant?

Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces. People with cystic fibrosis, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic. The fluid on airway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration; accordingly, scientists hypothesize that the high salt concentration is what makes the antibiotic ineffective.

Which of the following, if true, most strongly supports the scientists' hypothesis?

HIGH SALT CONCENTRATION (CAUSE)-----> ANTIBIOTIC TO BE INFFECTIVE(EFFECT)

Things to keep in mind when strengthening A CAUSAL RELATION

A. Eliminate any alternate causes for the stated effect
Because the author believes there is only one cause (the stated cause in the argument), eliminating other possible causes strengthens the conclusion.

B. Show that when the cause occurs, the effect occurs
Because the author believes that the cause always produces the effect, any scenario where the cause occurs and the effect follows lends credibility to the conclusion.This type of answer can appear in the form of an example.

C. Show that when the cause does not occur, the effect does not occur
Using the reasoning in the previous point, any scenario where the cause does not occur and the effect does not occur supports the conclusion. This type of answer also can appear in the form of an example.

D. Eliminate the possibility that the stated relationship is reversed
Because the author believes that the cause and effect relationship is correctly stated, eliminating the possibility that the relationship is backwards (the claimed effect is actually the cause of the claimed cause) strengthens the conclusion.

E. Show that the data used to make the causal statement are accurate, or eliminate possible problems with the data
If the data used to make a causal statement are in error, then the validity of the causal claim is in question. Any information that eliminates error or reduces the possibility of error will support the argument.


Now let us look at the answer choices :

A) This does not show any relationship between salt concentration and the functioning of the antibiotic.

B) Though the antibiotic was capable in killing bacteria in an environment where the salt concentration was unusually low, the choice does not say that in an environment where the salt concentration is high, the antibiotic will not be able to kill the bacteria.

C) CORRECT: since the environment created here has normal salt concentration and not high salt concentration, the point that high salt concentration in the lungs of such individuals is what has resulted in the antibiotic not being affected, is supported. Shows that when the cause does not occur, the effect does not occur

D)This does mention the cause for the malfunctioning of the antibiotic in patients of cystic fibrosis.

E) The answer does not strengthen the point that high salt concentration stops the antibiotic from killing bacteria.

VeritasKarishma - is B a contra positive ?

No, it isn't. Negation of high is "not high". It is not "unusually low".

On a side note, I don't subscribe to such terminology for GMAT CR. It makes things unnecessarily complicated. We are discussing what happens when salt concentration is high. When I read "low concentration," I did not give it another glance since it is irrelevant. Too much terminology confuses people and makes them waste time.

Healthy lungs produce a natural antibiotic that kills harmful bacteria.
People with cystic fibrosis, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic.
The fluid in their lungs has an abnormally high salt concentration

Hypothesis: the high salt concentration is what makes the antibiotic ineffective.

We need to support the hypothesis. So we need to say that it is the high concentration of salt that makes antibiotic ineffective.

(A) When the salt concentration of the fluid on the airway surfaces of healthy people is raised artificially, the salt concentration soon returns to normal.

Doesn't tell us what happens to bacteria. Not useful.

(B) A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.

We don't care what happens in low concentrations env.

(C) When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.

Correct. So it seems the tissue maintains its capability to fight bacteria in all other ways. Only the high salt concentration is the problem. So when placed in normal salt concentration, the tissue fights bacteria. This strengthens that high salt is responsible for making antibiotic ineffective.

(D) Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.

Irrelevant.

(E) High salt concentrations have an antibiotic effect in many circumstances.

Irrelevant. We need to know what it is doing in this circumstance.

Answer (C)

but isnt the word 'resist' different from killing.....maybe resisted but not killed.confused:|

You're right that the word "resist" is different than the word "kill," but that's not enough to dismiss (C) out of hand. You have to think through how the information in (C) impacts the scientists' hypothesis.

In healthy lungs, a certain antibiotic kills harmful bacteria. But notice that this language is softened later in the argument -- people with cystic fibrosis (CF) can't "fight off" the bacteria, even though they have the same antibiotic. Scientists hypothesize that an "abnormally high" salt concentration causes this issue.

So, how does (C) impact the argument?
If lung tissue can resist harmful bacteria when salt levels are normal, then (C) points the finger at high salt levels as the problem. Maybe if the salt levels were normal, people with CF would be able to fight off the bacteria.

Does this prove that normal salt levels would allow CF patients to kill harmful bacteria? No, it doesn't. However, it certainly supports the argument that the abnormally high salt levels are to blame for CF patients being unable to fight off the bacteria.

Since the question asks us which answer choice most strongly supports the scientists' hypothesis, (C) is the correct answer.

I hope that helps!

I see people saying C is the answer. My question to them is - Is there no difference between "resist" and "kill"?
While in B, I can understand that changing salt concentration has helped to "kill" the bacteria.
Please help.

Thanks for the explanation. Option C talks about "When lung tissue from people with cystic fibrosis" - It is already mentioned in the stimulus that People with cystic fibrosis already have high concentration of salts in the fluid on airway surfaces. What difference does it make if such a tissue is placed in a solution with normal concentration? Do we have to assume here that the high salt fluid on airway surfaces of such a lung tissue will be diluted to normal concentration after placing in the solution?

C. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.
- This chioce supports the conclusion that high salt concentration in people with cystic fibrosis renders the antibiotic useless.

Answer: C