Researchers studied 648 dementia patients with an average age of 66 years, and found that patients with two languages had dementia about four and a half years later than patients with only one language. It can delay the onset of dementia. Which of the following, if true, would most challenge the above conclusions? ["Many people's speech function declines significantly after suffering from dementia.", 'People with good brain cognitive function can speak bilingual, and good cognitive function can delay dementia.', 'Quite a few people have only one language, but never have symptoms of dementia throughout their lives.', 'People with three or more languages have dementia later than two.']