We are asked which of the following pairs "MUST have EXACTLY one symptom in common." This is different from rule #3, which states that J and L have "AT LEAST one symptom in common."
However, J and L could both have symptoms H and S, in which case they would have two symptoms in common. This would satisfy Rule #3 because it would be at least one shared symptom, but it would fail to satisfy the question because it would not be *exactly* one shared symptom. Since a possibility exists in which J and L could have two symptoms in common, they do not always have to have exactly one. Therefore, (A) is eliminated.
Does that make sense? Please let us know if you have additional questions and best of luck with your studies!
