The symptoms of mental disorders are behavioral, cognitive, or emotional problems. Some patients with mental disorders can be effectively treated with psychotherapy. But it is now known that in some patients mental disorders result from chemical imbalances affecting the brain. Thus these patients can be effectively treated only with medication that will reduce or correct the imbalance. The argument depends on assuming which one of the following? ['Treatment with medication always shows faster results for patients with mental disorders than does treatment with psychotherapy.', 'Treatment with psychotherapy has no effect on mental disorders other than a reduction of the symptoms.', 'Treatment by psychotherapy can produce no effective reduction in or correction of chemical imbalances that cause mental disorders.', 'Most mental disorders are not the result of chemical imbalances affecting the brain.']