题目选项分析

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?

【选项】The dental researchers could not discover why toothbrush contamination usually occurred only after toothbrushes had been used for four weeks.