Option D

Hello @Minerva, Let's take a closer look at the last sentence: Premise: Phosphate pollution from the municipal wastewater treatment plant decreased significantly in the last year. Conclusion: Some residents did switch to phosphate-free detergent. D is incorrect, because the percentage of total phosphate pollution that comes from the treatment plant is irrelevant. The author is only looking at one factor to draw the conclusion: the amount of phosphate pollution from the plant this year as compared to last year.

But if there are waste water from some of the residents being treated at other plants, wouldn't that weaken the author's argument?