Let's first examine the stimulus carefully. This stimulus presents an argument; the conclusion is "treatments that raise the concentration of magnesium in the blood would provide an effective cure for the fatigue involved in" chronic fatigue syndrome. What premises are provided? (1) chronic fatigue is always associated with lower than normal concentrations of magnesium in the blood, and (2) mal-absorption of magnesium from the digestive tract to the blood is often associated with some types of fatigue.
The question stem asks you to identify a weakness in this argument.
The premises in the stimulus show a correlation between fatigue and low blood magnesium, but the conclusion posits a cause & effect relationship between the two.
Answer choice (B) articulates this problem: that the argument "offers no evidence that fatigue itself does not induce lowered concentrations of magnesium in the blood." In other words, it's possible that fatigue is the cause of low blood magnesium, not the other way around.
Answer choice (A) focuses on the two premises rather than the link between the premises and the conclusion. You are asked, however, to point out the weakness in the argument - in the link between the two premises together, on the one hand, and the conclusion, on the other. Unlike (B), answer (A) does not do this.
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