题干

1931年英国物理学家狄拉克从理论上预言:存在只有一个磁极的粒子即“磁单极子”.1982年美国物理学家卡布莱设计了一个寻找磁单极子的实验,仪器的主要部分是一个由超导体组成的线圈,超导体的电阻为零,一个微弱的电动势就可以在超导线圈中引起感应电流,而且这个电流将长期维持下去,并不减弱,他设想,如果一个只有N极的磁单极子从上向下穿过如图所示的超导线圈,那么从上向下看,超导线圈上将出现(   )

A:先是逆时针方向的感应电流,然后是顺时针方向的感应电流

B:先是顺时针方向的感应电流,然后是逆时针方向的感应电流

C:顺时针方向持续流动的感应电流

D:逆时针方向持续流动的感应电流

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D

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    National Rankings: Best High Schools

    The U.S. News rankings include data on more than 22,000 public high schools in 50 states and the District of Columbia. Schools were awarded gold, silver or bronze medals based on their performance on state assessments and how well they prepare students for college.

    About the Awards

    Gold—Top 500 Schools based on highest college readiness.

    Silver—High-performing schools based on lower college readiness.

    Bronze—High-performing schools based on state exam performance,  listed alphabetically.

    Award Distribution

    Gold 2.4%

    Silver 10.3%

    Bronze 16.8%

    No Medal 70.5%

    HOW We Determine the Awards

    STEP 1 Students perform better than expected in their state.

    We looked at reading and math results for students on each state's proficiency(水平) tests and then factored in the percentage of economically disadvantaged students, who tend to score lower.

    STEP 2 Disadvantaged students perform better than state average.

    We compared each school's math and reading proficiency rates for disadvantaged students--black, Hispanic and low-income—with the state wide results for these student groups and then selected schools that were performing better than their state averages.

    STEP 3 Student graduation rates meet or are greater than a national standard.

    We left out schools from consideration if their graduation rates were lower than 75 percent--a starting point that is higher than a federal law that requires states to give extra resources to schools below 67 percent.

    STEP 4 Students are prepared for college-level coursework.

    We calculated a College Readiness Index,which is based or the school's AP participation rate and how well the students did on those tests. Tiebeakers were used to determine ranks of schools that achieved the same College Readiness Index.