题干

如图所示,烧杯的底面积是200cm2,里面装有一定量分水(图甲),用弹簧测力计吊着未知物体,先将物体浸没在水中(图乙),水位升高到B出,示数是18N;再将物体缓慢提出,使水位下降到AB的中点C处,示数是23N(不计物体带出的水)。g取10N/kg,下列判断中正确的是(  )

①物体的质量是2.8kg;②物体的体积是1×103m3;③物体浸没时受到的浮力是15N;④从乙到丙,水对烧杯底面的压强减小了260Pa。

A:只有①、②、④正确

B:只有①、②、③正确

C:只有①、②正确

D:只有③、④正确

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C

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    Some educators told us that more years of school could help students get higher scores on intelligence tests. That was a finding of a study of teenage males in some countries. Now, another research shows that physical activity may help students do better in their classes.

    The research comes as educators in some countries are reducing time for activities like physical education. They are using the time instead for academic(学术的)subjects like math and reading. The studies appeared between 2008 and 2014. They included more than 55, 000 children, aged 6 to 18.

    Amika Singh:“ According to the results of our study, we can conclude that being physically active is beneficial for academic performance. There are, first, Physiological explanations, like more blood flow, and so more oxygen to the brain. Being physically active means there are more hormones(荷尔蒙)produced like endorphins(内啡肽). And endorphins make your stress level lower and your mood improved, which means you also perform better.”

    Also, students taking part in organized sports learn rules and how to follow them. This could improve their classroom behavior and help them keep their mind on their work.

    The study leaves some questions unanswered, however. Ms. Stash says it is not possible to say whether the amount or kind of activity affected the level of academic improvement. This is because of differences among the studies.

    The researchers said they found only two high-quality studies. They needed more high-quality studies to confirm (证实) their findings. They also pointed out that “results for other parts of the world may be quite different.”

    Still,the general finding was that physically active kids are more likely to do better in school. Ms. Singh says schools should consider that finding before they cut physical education programs. Her paper on “Physical Activity and Performance at School” is published in the Archives of Pediatric and Adolescent Medicine.