A study of adults who suffer from migraine headaches revealed that a significant proportion of the study participants suffer from a complex syndrome characterized by a set of three symptoms. Those who suffer from the syndrome experienced excessive anxiety during early childhood. As adolescents, these people began experiencing migraine headaches. As these people approached the age of 20, they also began to experience recurring bouts of depression. Since this pattern is invariant, always with excessive anxiety at its beginning, it follows that excessive anxiety in childhood is one of the causes of migraine headaches and depression in later life. The reasoning in the argument is vulnerable to criticism on which one of the following grounds? ['It fails to rule out the possibility that all of the characteristic symptoms of the syndrome have a common cause.', 'It does not specify the proportion of those in the general population who suffer from the syndrome.', 'It does not establish why the study of migraine sufferers was restricted to adult participants.', 'It makes a generalization that is inconsistent with the evidence.']