C??

1) healthy lungs produce natural antibiotic

2) antibiotic protects lungs from infection by routinely killing harmful bacteria on airway surface

3) people with cystic fibrosis are unable to fight off bacteria, even when lungs produce normal amounts of antibiotics

4) Fluid on airway surfaces in lungs of people with cystic fibrosis has an abnormally high salt concentration

5) scientist hypothesize that high salt concentration is what makes the antibiotic ineffective <-- conclusion

We want an answer that supports the conclusion

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.
- Does not say salt concentration renders antibiotic ineffective

B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
- Only supports low concentration salt premise

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.

D. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.
- Tells us nothing

E. High salt concentrations have an antibiotic effect in many circumstances.
- Negative effect or positive effect. Not stated here. E is out.

C it is.

clearly C for me

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

In my opinion :
B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
no information at all, we just know the antibiotic can kill bacteria when there is a low concentration of salt however the issue raised here is only when there is a high salt concentration. So it's totally useless to know the action of the antibiotic in a low salt environnement. Moreover, choice B doesn't clearly precise whether the sample of the antobiotic is coming from people with cystic fibrosis or not...

Agree with (C)

It is C because all the three necessary are covered.. salt solution, cystic fibrosis and effects of the same tissue from people who had cystic fibrosis in normal levels of salt solution..

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