Healthy lungs produce a natural antibiotic that protects them from inf

Yeah, D provides alternative explanation for the ineffectiveness of the antibiotic undermining the conclusion

Cheers,
Unplugged

we have to show that
ppl with cystic fibrosis are unable to fight off harmful bacteria even when the salt concentration is reduced


IMO D

Though a bit late.....
IMO D...

Option D provides the best alternative explanation that its not the salt level, but the lungs itselfs which are causing the problem

A - Strengthening
B-2nd best answer: People with cystic fibroses, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic.: Hence, No
C-Okay. Additional information not relevant to the question
D- Perfect . Answer
E-Out of scope of the question

really surprised at the tagged level of this question, doesnt seem like a sub 500 level question. Found it quite confusing

Go for diagramming it in your own conventional way U will find it much easier to solve -

Understanding the argument -
Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces. - Fact
People with cystic fibroses, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic. - "however" and "even though" introduces contrast. Fact
Since the fluid on airway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration, scientists hypothesize that in high salt environments, the antibiotic becomes ineffective at killing harmful bacteria.- supporting premise + Main conclusion

Option Elimninaiton - Weakener
The scope of the argument is to weaken the conclusion that "in high salt environments, the antibiotic becomes ineffective at killing harmful bacteria." Meaning its the high salt environment that makes the antibiotic ineffective. We need to weaken this cause. X causes Y. Way to weaken is
1. not X but we have Y
2. X but not Y
3. Alternate cause Z.

(A) Healthy lungs in which the salt concentration of the airway-surface fluid has been substantially increased are able to reestablish their normal salt concentration within a relatively short period of time. - It essentially says that "Healthy lungs can reestablish their normal salt concentration." The scope of the argument is to weaken the conclusion that "in high salt environments, the antibiotic becomes ineffective at killing harmful bacteria." This means it's the high salt environment that makes the antibiotic ineffective. We need to weaken this cause. At best, this is out of scope.

(B) The antibiotic produced by the lungs is effective at killing harmful bacteria even when salt concentrations are below levels typical of healthy lungs. - we need a case with "high salt concentrations." "Low levels of salt concentrations" are out of scope.

(C) The salt concentration of the airway-surface fluid in the lungs of people who suffer from cystic fibrosis tends to return to its former high levels after having been reduced to levels typical of healthy lungs. - It essentially says that "the salt concentrations tend to return to their former high levels." The scope of the argument is to weaken the conclusion that "in high salt environments, the antibiotic becomes ineffective at killing harmful bacteria." This means it's the high salt environment that makes the antibiotic ineffective. We need to weaken this cause. At best, this is out of scope.

(D) The lungs of people who suffer from cystic fibrosis are unable to fight off harmful bacteria even when the salt concentration is reduced to levels typical of healthy lungs. - It says Y is still there even when X is not. So, there must be an alternate cause. Ok.

(E) The salt concentration in the airway-surface fluid of people whose lungs produce lower-than-average amounts of the antibiotic is generally much lower than that typical of healthy lungs. - "People whose lungs produce lower-than-average amounts of the antibiotic " are out of the scope of this argument.