Many scientific studies have suggested that taking melatonin tablets can induce sleep. But this does not mean that melatonin is helpful in treating insomnia. Most of the studies examined only people without insomnia, and in many of the studies, only a few of the subjects given melatonin appeared to be significantly affected by it. Which one of the following, if true, most strengthens the argument? ['None of the studies that suggested that taking melatonin tablets can induce sleep examined a fully representative sample of the human population.', 'Several people who were in control groups and only given placebos claimed that the tablets induced sleep.', 'If melatonin were helpful in treating insomnia, then every person with insomnia who took doses of melatonin would appear to be significantly affected by it.', 'In the studies that included subjects with insomnia, only subjects without insomnia were significantly affected by doses of melatonin.']