GMAT Critical Reasoning (CR) | Re: Dental researchers recently discovered that tooth-brushes

Dental researchers recently discovered that tooth-brushes can become contaminated with bacterial that cause pneumonia and strep throat. They found that contamination usually occurs after toothbrushes have been used for four weeks. For that reason, people should replace their toothbrushes at least once amonth.

Which of the following, if true, would most weaken the conclusion above?

(A)