A housing society says that the water filters it fitted in every flat last year have reduced stomach infections among the residents.
Which of the following, if true, most strengthens the society’s claim?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Tests show that the filtered water has far fewer disease-causing germs than the tap water the residents drank earlier.
A claim that one thing caused another is strengthened by a fact that shows how the cause produced the result, and weakened by a fact that points to some other cause. Option (3) shows the filters at work: the residents now drink water with far fewer germs, and germs in water are a common cause of stomach infections.
Why the other options are incorrect
Option (1) Water filters of the same make are sold in many cities. — Incorrect. Where the filters are sold says nothing about infections in this society.
Option (2) Last year the society also stopped residents from buying food from roadside stalls. — Incorrect. This gives another reason for fewer infections, so it weakens the claim.
Option (4) The filters were fitted by a company that the society had not used before. — Incorrect. Who fitted the filters has no bearing on whether they reduced infections.