Critical Reasoning

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?

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.
B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
C. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.
D. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.
E. High salt concentrations have an antibiotic effect in many circumstances.

OA is C
Can OA be specifically true? Lets ponder on option D, an Expert help is needed here. Thanks

Roland2rule wrote: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?

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.
B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
C. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.
D. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.
E. High salt concentrations have an antibiotic effect in many circumstances.

OA is C
Can OA be specifically true? Lets ponder on option D, an Expert help is needed here. Thanks

Conclusion: In people with cystic fibrosis, it's the high salt concentration that renders the antibiotic ineffective.
If it's the salt concentration, and not some other factor, that's the culprit, then presumably, if we were to normalize the salt content, the antibiotic would then be effective. This is what C gives us.

D doesn't the address the issue of whether salt concentration is the decisive factor in undermining the effectiveness of the antibiotic in those with cystic fibrosis. (Notice that words such as "salt" and "cystic fibrosis" both show up in the correct answer, and neither shows up in D, a good litmus test for whether a prospective answer remains within the scope of the argument.)