12 . Studies of the causes of ADHD have found that environmental toxins, such as food additives and lead,
(A) are usually the cause in those children with an organic basis to their ADHD
(B) cause subtle attentional problems, but not to the degree of ADHD
(C) are more likely to cause attentional problems without hyperactivity
(D) do not explain more than a small percentage of cases

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