Ordinary mountain sickness, a common condition among mountain climbers, and one from which most people can recover, is caused by the characteristic shortage of oxygen in the atmosphere at high altitudes. Cerebral edema, a rarer disruption of blood circulation in the brain that quickly becomes life-threatening if not correctly treated from its onset, can also be caused by a shortage of oxygen. Since the symptoms of cerebral edema resemble those of ordinary mountain sickness, cerebral edema is especially dangerous at high altitudes. Which one of the following is an assumption on which the argument depends? ['Unlike cerebral edema, ordinary mountain sickness involves no disruption of blood circulation in the brain.', 'The treatment for ordinary mountain sickness differs from the treatment for cerebral edema.', 'Most people who suffer from ordinary mountain sickness recover without any special treatment.', 'Cerebral edema can cause those who suffer from it to slip into a coma within a few hours.']