问题 选择题

下列做法可实现预期目标的是(  )

A.寻找新的催化剂使水变成汽油

B.常用自来水冲洗自行车可防止生锈

C.用氮气作焊接金属的保护气

D.向硬水中加明矾净水剂后得到纯水

答案

A、水由氢氧两元素组成,不含组成汽油的C元素,而催化剂只是改变反应速度的一类物质,并不能使反应产生新的元素.水变油不符合质量守恒定律.所以此做法不可能达到预期目标;

B、铁在潮湿的空气中会发生锈蚀,所以用自来水冲洗自行车非但不能防止生锈还会造成锈蚀更严重.做法不能达到预期目标.

C、氮气化学性质稳定,很难与其他物质反应,用氮气把金属与氧气隔绝防止金属与氧气发生反应,保护被焊接的金属.做法可以达到预期目标.

D、明矾是一种絮凝剂,可以吸附水中不溶性的固体小颗粒而加快固体的沉降,并不能与硬水中的钙、镁离子发生作用,起不到使硬水变成纯水的作用.不能达到预期目标.

故选C

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PALO ALTO, California------"Switching off the television may help prevent children from getting fatter------ even if they do not change their diet or increase the amount they exercise," US researchers said last week.

  A study of 192 third and fourth graders, generally aged eight and nine, found that children who cut the number of hours spent watching television gained nearly two pounds(0.9kg) less over a one-year period than those who did not change their television diet.

  "The findings are important because they show that weight loss can only be the result of a reduction in television viewing and not any other activity," said Thomas Robinson, a pediatrician(儿科专家) at Stanford University.

  "American children spend an average of more than four hours per day watching television and videos or playing video games, and rates of childhood being very fat have doubled over the past 20 years," Robinson said.

  In the study, presented this week to the Pediatric Academic Societies‘ annual meeting in San Francisco, the researchers persuaded about 100 of the students to reduce their television viewing by one-quarter to one-third.

  Children watching fewer hours of television showed a significantly smaller increase in waist size and had less body fat than other students who continued their normal television viewing, even though neither group ate a special diet or took part in any extra exercise.

  "One explanation for the weight loss could be the children unstuck to the television may simply have been moving around more and burning off calories," Robinson said.

  "Another reason might be due to eating fewer meals in front of the television. Some studies have suggested that eating in front of the TV encourages people to eat more," Robinson said.

小题1:The author tries to tell us in the first two paragraphs that ________.

A.children will get fatter if they eat too much

B.children will get thinner if they eat less

C.children will get fatter if they spend less time watching TV

D.children will get fatter if they spend more time watching TV小题2:According to the passage, the time American children usually spend on watching TV_____.

A.is more than four hours a day

B.is less than four hours a day

C.doubled in the last twenty years

D.is more than on any other activities小题3: The time children spend on TV viewing every day is suggested to be about ________.

A.six hours

B.eight hours

C.three hours

D.one hour小题4:Which one of the following is right?

A.Children usually eat fewer while watching TV.

B.Children usually eat more while watching TV.

C.Children eat the same amount of meals while watching TV.

D.Children usually eat nothing while watching TV.小题5:Why can watching TV increase kids’ weight according to the passage?

  A. They usually eat more while watching TV.

  B. They burn off fewer calories.

  C. They change their diet while watching TV.

D. Both A and B.