单选题：根据以下材料回答第题：　　Children have been said to have brain-injured c

根据以下材料回答第题：
　　Children have been said to have brain-injured child syndrome, hyperactive (极度活跃的) child syndrome and attention-deficit disorder (ADD).As early as the 1940's, psychiatrists have (1) children with hyperactivity and extremely distracted and impulsive. These frequent name changes show how (2) researchers are about the causes of the problem and the diagnostic (诊断) (3) for the disorders.
　　In the past several years．the people who study attention deficit hyperactivity disorder(ADHD)have (4) to clarify the disorder’s causes and (5)，and have reasons to believe mat it may be (6)．Today’s view on this disorder is very different from the beliefs of just a few years ago．
　　Researchers are finding that ADHD is nota disorder of just attention，but is from a developmental (7) in the brain’s circuitry and self-control．The loss of self-contr01 (8) 0ther important brain functions compulsory foll (9) attention．ADHD (10) two sets of symptoms：inattention and a (11) of hyperactive and impulsive behaviors．Most children are more active and (12) than adults are．Children are more inconsistent．(13) by brief actions．The younger the child，the (14) aware they are．
This behavior is a (15) 0f a problem when it is displayed more in them than in their (16)．Boys are at least three times more (17) than girls to have ADHD．Studies have found that ADHD in boys outnumbers girls 9 to 1：this is because boys are genetically likely to develop disorders in thenervoussystem．These behaviors (18) 0ccur between the ages of 3 and 5．(19)．some children don’t start to develop the disorder until late childhood or earlyadolescence．Researchers are unable to find the reasons for this (20)．
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