7．causes 根据第二段第三句“Researchers have been looking for the cause or causes of attention deficit hyperactivity disorder.”可知答案。
8．short 根据第二段中“Researchers say they observed shortages in the brain’s reward system in patients with A．D.H.D.”可知答案。
9．low 根据第二段中“There was a lower concentration of dopamine markers in the brain of individuals with A．D.H.D. ”可归纳出答案。
10．increase/raise 根据文章最后一段的最后一句“He says they might be attempting to increase their dopamine levels to make up for the deficits in their reward system.” 可知答案。
