题干

科技小组的同学设计了如图甲所示的恒温水箱温控电路(设环境温度不高于20℃),由工作电路和控制电路组成.工作电路中的电热器上标有“220V2000W”的字样;控制电路中热敏电阻Rt作为感应器探测水温,置于恒温水箱内,其阻值随温度变化的关系如图乙所示,R1为滑动变阻器.电磁铁产生的吸引力F与控制电路中电流I的关系如图丙所示,衔铁只有在不小于3N吸引力的作用下才能被吸下.闭合开关S(不计继电器线圈的电阻).则正确的是(     )

A:电磁铁上端为N极

B:用电热器给恒温箱中100kg的水加热,正常工作20min时,水温由20℃升高到25℃,那么此时电热器的加热效率为87.5%

C:调节滑动变阻器,使R1=280Ω,为了把温度控制在25℃左右,设定在水温低于25℃时自动加热,在水温达到25℃时停止加热,那么控制电路的电源电压U应该是36V

D:设控制电路中电源电压U不变,将水箱中水温由25℃的恒温调节到30℃的恒温,应减小滑动变阻器接入电路中的阻值.

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B,C

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    “Daily Star, sir” called Jason, carrying some newspapers under his arm. The little boy had been running up and down the street, but there were still twenty1 left. His voice was almost gone and his heart was 2. The shops would soon close, and all the people would go home. He would have to go home too, carrying the papers3money. He had hoped to sell more papers tonight to make more money to buy a 4for his mother and some seeds for his bird. That was why he had bought the papers with all his money. He 5as he thought of his failure to sell all his papers.

    “You don't know the 6 of selling papers. You must shout, “Hot news! Bomb bursting!” another newsboy Chad told Jason. “7 it's not in the paper at all,” replied Jason. “Just run away quickly 8 they have time to see, and you'll 9 out and get your money,”Chad said.

    It was a new 10 to Jason. He thought of his bird with no11 and the cake he wanted to buy for his mother, but was12 that he would not tell a lie. Though he was13a poor newsboy, he had been 14 some good things.

    The next afternoon Jason went to the office for his papers15 .Several boys were crowding around Chad, who declared with a 16 smile that he sold six dozen the day before. He added that Jason 17 money because he would not tell a lie. The boy 18 at Jason. “You wouldn't tell a lie yesterday, my boy?” A gentleman at the office came up and patted Jason's shoulder 19 .”You're just the boy I am looking for.” A week later Jason started his new20. He lost sale of twenty papers because he would not tell a lie, but got a well-paid job because he told the truth.