Chronic fatigue syndrome, a condition that afflicts thousands of people, is invariably associated with lower-than-normal concentrations of magnesium in the blood. Further, malabsorption of magnesium from the digestive tract to the blood is also often associated with some types of fatigue. These facts in themselves demonstrate that treatments that raise the concentration of magnesium in the blood would provide an effective cure for the fatigue involved in the syndrome. The argument is most vulnerable to which one of the following criticisms? ['It fails to establish that lower-than-normal concentrations of magnesium in the blood are invariably due to malabsorption of magnesium.', 'It offers no evidence that fatigue itself does not induce lowered concentrations of magnesium in the blood.', 'It neglects to state the exact concentration of magnesium in the blood which is considered the normal concentration.', 'It ignores the possibility that, even in people who are not afflicted with chronic fatigue syndrome, concentration of magnesium in the blood fluctuates.']