The vomeronasal organ (VNO) is found inside the noses of various animals. While its structural development and function are clearer in other animals, most humans have a VNO that is detectable, though only microscopically. When researchers have been able to stimulate VNO cells in humans, the subjects have reported experiencing subtle smell sensations. It seems, then, that the VNO, though not completely understood, is a functioning sensory organ in most humans. Which one of the following, if true, most weakens the argument? ['Relative to its occurrence in certain other animals, the human VNO appears to be anatomically rudimentary and underdeveloped.', 'For many animal species, the VNO is thought to subtly enhance the sense of smell.', 'Certain chemicals that play a leading role in the way the VNO functions in animals in which it is highly developed do not appear to play a role in its functioning in humans.', "It is not known whether the researchers succeeded in stimulating only VNO cells in the human subjects' noses."]