Human skin gives off an array of gaseous substances, including carbon dioxide and lactic acid, both of which attract mosquitoes. However, neither of these two substances, whether alone or combined with one another, will attract mosquitoes as much as a bare human arm will, even in complete darkness, where a mosquito has no visual cues. Ttherefore, some other gaseous substance given off by human skin also attracts mosquitoes. The reasoning in the argument requires which one of the following assumptions? ['Mosquitoes are no more successful in finding a bare human arm in darkness than in light.', 'Human skin gives off gaseous substances in greater amounts during the day than during the night.', 'Mosquitoes are not attracted to humans by body heat.', 'Human skin never gives off any gaseous substances that repel mosquitoes.']