One study selected 80 young people aged 13 to 25 who had early symptoms of schizophrenia as subjects, and randomly selected half of them to take fish oil tablets for 3 months and the other half to take placebo for 3 months. After a 7-year follow-up study of the subjects, it was found that only 10% of the people taking the fish oil tablet group suffered from schizophrenia; while in the placebo group, as many as 40% of the people suffering from this disease. Therefore, the researchers concluded that taking fish oil tablets helps prevent adolescents with schizophrenia. If the following are true, which of the following can be used to challenge the researchers' conclusions? ['The number of subjects is still small, and the development of each subject in the 7 years is quite different.', 'Subjects have a large age span. At the beginning of the experiment, some people may have been young rather than young.', 'Fish oil tablets have been tried to treat adults with schizophrenia, but experiments have found no effect.', 'Subjects take fish oil tablets for a short period of time, which is not enough to form a stable and obvious effect.']