Dental researchers recently discovered that tooth-brushes can become contaminated with bacteria 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 a month.

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

• AThe dental researchers could not discover why toothbrush contamination usually occurred only after toothbrushes had been used for four weeks.  
• BThe dental researchers failed to investigate contamination of toothbrushes by viruses, yeasts, and other pathogenic microorganisms.  
• CThe dental researchers found that among people who used toothbrushes contaminated with bacteria that cause pneumonia and strep throat, the incidence of these diseases was no higher than among people who used uncontaminated toothbrushes.  
• DThe dental researchers found that people who rinsed their toothbrushes thoroughly in hot water after each use were as likely to have contaminated toothbrushes as were people who only rinsed their toothbrushes hurriedly in cold water after each use.  
• EThe dental researchers found that, after six weeks of use.greater length of use of a toothbrush did not correlate with a higher number of bacteria being present.