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 infectionPeople 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
KUDOS is the good manner to help the entire community.