Researchers recently discovered an unusually high concentration of harmful metals in the water supply of Dunbarton county. They further found that, with a costly new water filter system installed in their homes, residents were able to filter out almost all of the harmful metals in the water supply. For this reason, all the residents of Dunbarton county should have the filter system installed in their homes, regardless of expense.

If true, which of the following would most weaken the above conclusion?

A. Researchers could not discover how the harmful metals were entering the water supply.

B. The increased levels of harmful metals were insufficient to result in significant health effects.

C. The researchers failed to investigate the possibility of harmful microorganisms and other contamination in the county's water supply.

D. The researchers found that people who tried to purify their water by boiling it were just as likely to ingest increased levels of harmful metals as people who drank straight from the tap.

E. Researchers found that, over time, the levels of harmful metals in the water supply did not vary significantly and remained consistently high.

"For this reason" indicates that there is only one main premise.
There are 2 or 3 patterns in option choices.