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二冲程摩托车发动机可燃混合气包括（）。
Passage1Children who spend more than two hours a day at a computer or watching television are more likely than others to have mental problems，scientists say.Researchers found that 11-year-olds who spent several hours in front of a screen each day did worse on mental health tests，no matter how much physical exercise they got. The University of Bristol study，published in Pediatrics，involved more than 1，000 children aged about 10.They also had the kids fill out questionnaires designed to gauge the kids' emotional well-being and behavior.The questionnaires contained 20 questions covering five sections-emotional difficulties，conduct problems，hyperactivity or inattention，friendships and peer groups and problems relating to friends and peer groups.The study found no direct evidence that sitting in front of a screen actually causes mental health problems. Instead，it suggests that children with difficulties，such as extreme shyness，are more likely to choose TV or computer games than sociable activities. In the study，children were asked whether they agreed，disagreed or somewhat agree with a list of statements，including，"I generally play alone or keep to myself" and，"I am often unhappy or tearful".They were also asked how much exercise they took and how long they spent at a TV or computer screen.According to the study，children who spent more than two hours a day at a screen had a 60% higher risk of mental problems than children who spent fewer viewing hours. The risk was only slightly higher in children who did little or no exercise."Physical activity is good for health in many ways，but it can't make up for long hours of screen viewing. Parents should consider limiting their children's screen viewing to no more than 2 hours a day，”said the study organizer，Angie Page.Previous studies have raised concerns that watching too much television can affect children's behavior in later life. A Canadian study found that those who watched most TV as young children performed worse at school, ate more junk food and had more trouble concentrating.How did the researchers carry out the study?
（）是以建筑物或构筑物各个分部分项为对象编制的定额。
如图所示该安全标志表示的意思是（）。
婚姻成立还需要实质要件，这是婚姻成立的关键。客户的婚姻关系如果不符合()的实质要件，可能非但不受法律的保护，甚至已经成立的婚姻也面临着被撤销的风险。
永续年金可视作期限无限的普通年金，终值与现值的计算可在普通年金的基础上求得。
服务的核心本质是（）
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关于法规与政策，下列描述错误的是（ ）。
肾功能不全时，机体钙吸收减少的机制是（）
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