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Weaken—CE, #%. The correct answer choice is (D)
The key to this author’s argument is the belief that Albritten’s water will contain at least 250 milligrams of salt per liter within the next few decades, making it unpalatable. This conclusion is based on the fact that Albritten’s water currently contains 100 milligrams of salt, significantly more than that of a nearby area. Of course, the belief that the salt content will increase by at least 150 mg stems from the assumption that before the road salting began it must have been much lower than the current 100 mg (that is, the author obviously assumes that it has already risen substantially in order to conclude that it will continue to rise substantially in the future). To weaken this argument, the correct answer choice should attack this assumption and show that the salting has not had much (or any) effect on the present salt levels. That would make it unlikely that the content would rise 150 mg in the next few decades.
Answer choice (A): The argument is about palatability (taste), not safety, so this answer choice is incorrect.
Answer choice (B): This answer choice strongly supports the conclusion, as the water would certainly be unpalatable if it reaches 400 mg/liter (since 250 is said to be the threshold).
Answer choice (C): The motive for salting the roads is not important to the argument; the correct answer must attack the water’s salt content and the presumed increase.
Answer choice (D): This is the correct answer choice. If the salt content has only risen 10 mg in the past 20 years (from 90 to 100), then it is quite unlikely that it will rise 150 in the next 20 years.
Answer choice (E): Since the argument concludes that the salt content will reach 250 if salting continues “at the present rate,” this answer choice does not address the argument.