题干

有5个球,其中2个一样的黑球,红、白、蓝球各1个,现从中取出4个球排成一列,则所有不同的排法种数是(   )

A:72

B:60

C:120

D:54

上一题 下一题 0.0难度 选择题 更新时间:2015-03-21 05:54:39

答案(点此获取答案解析)

B

同类题3

阅读理解

    I don't think there is anything more important than health. “Health is the greatest wealth,” wise people say. You can't be good at your studies or work well when you are ill.

    If you have a headache, toothache, backache, earache or bad pain in the stomach, if you complain of a bad cough, if you run a high temperature and have a bad cold, or if you suffer from high or low blood pressure, I think you should go to the doctor. The doctor will examine your throat, feel your pulse, test your blood pressure, take your temperature, sound your heart and lungs, test your eyes, check your teeth or have your chest X-rayed. After that, he will advise some treatment, or some medicine. The only thing you have to do is to follow his advice.

    Speaking about doctor's advice, I can't help telling you a funny story. An old gentleman came to see the doctor. The man was very ill. He told the doctor about his weakness, memory loss and serious problems with his heart and lungs. The doctor examined him and said there was no medicine for his disease. He told his patient to go to a quiet place for a month and have a good rest. He also advised him to eat a lot of meat, drink two glasses of red wine every day and take long walks. In other words, the doctor advised him to follow the rule: "Eat with pleasure, drink with pleasure and enjoy life as it is." The doctor also said that if the man wanted to be well again, he shouldn't smoke more than one cigarette a day. A month later the gentleman came into the doctor's office. He looked cheerful and happy. He thanked the doctor and said that he had never felt a healthier man." But you know, doctor," he said, “it's not easy to begin smoking at my age.”

同类题4

课外阅读二:

地球将会变得有多热?

蒋葳

    今年初,世界气象组织(WM0)发布报告称,2015年成为有记录以来最热的一年。不久前,科学家又表示2016年将会是历史上最热的一年。自有记录以来,世界十大最热年都发生在1998年之后,8个在2005年之后,其中,2011~201 5年是历史上最热的5年。就目前而言,全世界的温度似

乎一直在上升。地球究竟能变得有多热?人类导致的变暖效应究竟是否存在上限?

    地球变热的原因——温室效应

    纵观地球46亿年的历史,地球经历了无数次温度波动,但最终总能回归到大约相同的温度范围,这是因为它存在控制自身温度的机制。其中一个关键机制就是温室效应。空气中的温室气体,主要是二氧化碳、甲烷和水蒸气,会围困来自太阳的辐射,就像一个隔热层包裹着地球。没有温室效应,地球平均温度将低达一l8℃,地表将覆盖冰,我们所知的生命将无法存活。

    很明显温室效应是个好现象,但它也可能过犹不及。在地球上人类存在的时间非常短,然而我们却成为地球气候变化最重要的驱动因素。通过燃烧化石燃料和砍伐树木,我们向大气层释放了越来越多的二氧化碳,而这已经导致温度上升。在2 000~2010年的l0年间,我们释放二氧化碳的速率已经超过了前10年的4倍,且这种上升趋势并没有任何逆转的迹象。问题在于,这些额外的温室气体在未来几十年或者几个世纪内将把地球加热到什么程度?

    地球升温4℃将是“灾难性的”

    为了预测未来地球的状态,科学家们建立了电脑模型模拟地球气候变化。这些模型可以估计当释放了特定量的温室气体,气候将如何发生变化。2013—2014年,政府间气候变化专门委员会(简称IPCC)将这些预测整理成报告。结果表明,如果温室气体释放量像过去50年一样继续上升,那么到本世纪中叶,地球温度将比工业化前高出l.5℃,到本世纪末地球温度将比工业化前高出4℃。

    全球气温上升l℃,不少林荫、田野就会变成干枯的荒漠;增加2℃,众多珊瑚礁就可能消失:升温3℃,夏季时北极圈将失去积雪,亚马逊雨林会逐渐枯萎;提高4℃,威尼斯等沿海城市将被完全淹没……英国《自然》杂志曾刊文称,到2100年,地球可能至少升温4℃。对人类来说,2℃是地球升温的“警戒线”,4℃则会是“灾难性的”。

    地球会成为超级大火炉吗?

    金星有时候被称为地球的另一个“邪恶的双胞胎”,这是因为,大小、质量和密度都与地球相近的金星,被认为历史上也曾拥有与地球类似的环境,但如今的它却是一个十足的炼狱:金星是太阳系中表面温度最高的行星,达到46 0℃,生命在金星上根本无从立足。

    科学家认为,这一切的罪魁祸首就是“失控的温室效应”。人们不禁担忧:我们所居住的地球是否也可能面临同样的命运?

    新的能量平衡计算结果认为,目前地球上热呼呼的大气吸收的热量比散发出的热量更多。那么,温室效应失控现象会在地球上发生吗?根据科学家的计算,地球要变成一个失控的“温室”,大气中的二氧化碳浓度必须达到大约30000ppm(1ppm即百万分之一),但就算我们烧光了地球上所有可用的化石燃料储量,也不可能出现这种情况。

【摘自《百科知识》2016.10】