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 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.