People with higher-than-average blood levels of a normal dietary by-product called homocysteine are twice as likely to be diagnosed with Alzheimer' s disease as are those with average or below-average homocysteine levels. Thus, it is likely that the risk of developing Alzheimer' s disease could be reduced by including in one' s diet large amounts of B vitamins and folic acid, which convert homocysteine into substances known to have no relation to Alzheimer' s disease. Which one of the following, if true, most seriously weakens the argument? ["Alzheimer's disease tends to increase the levels of homocysteine in the blood.", "Many Alzheimer's patients have normal homocysteine levels.", 'B vitamins and folic acid are not metabolized by the body very efficiently when taken in the form of vitamin-mineral supplements.', "People whose relatives contracted Alzheimer's disease are much more likely to develop Alzheimer's than those whose relatives did not."]