题干

下列化学用语中,数字“2”能用来表示分子个数的是(  )

A:2N

B:2Mg2+

C:2CO

D:O2

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C

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    The bed should be reserved as a place for sleep, but people tend to read an iPad a lot in bed before they go to sleep.

    Charles Czeisler, a professor at Harvard Medical School, and his colleagues got a small group of people for an experiment. For five days in a row, the people read either a paper book or an iPad for four hours before sleep. Their sleep patterns were monitored all night. Before and after each trial period, the people took hourly blood tests to paint a day-long picture of just how much melatonin (褪黑激素) was in their blood at any given time.

    When subjects read on the iPad as compared to the paper books, they reported feeling less sleepy at night and less active the following morning. People also took longer to fall asleep on the iPad nights, and the blood tests showed that their melatonin secretion (分泌) was delayed by an hour and a half.

    The researchers conclude in today's journal article that given the rise of e-readers and the increasingly widespread use of e-things among children and adolescents, more research into the long-term consequences of these devices on health and safety is urgently needed. Czeisler and colleagues go on, in the research paper, to note:“Reading an iPad in bed may increase cancer risk.”

    However, software has been developed that can reduce some of the blue light from the screens of phones and computers according to time of day, and there are also glasses that are made to filter (过滤) short wavelengths. While they seem like a logical solution for the nighttime tech users, it needs more research.

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海鸥乔纳森(节选)

    ①早晨,初升的太阳照耀着恬静的海面.荡漾的微波闪着金光。

    ②离岸一英里的海上.一只渔船随波逐浪地前进,这是吃早饭的信号,近千只海鸥飞来,相互追逐着争食吃。又一个忙碌的日子开始了。

    ③但在远离渔船和海岸的地方.海鸥乔纳森·利文斯顿独自在练习飞行。在百英尺的上空.他伸下两只带蹼的脚,仰起嘴.使劲儿弯着翅膀。翅膀一弯.就可以放慢速度。而现在,他越飞越慢了,慢得几乎听不到耳边的风声,慢得连脚下的大海也仿佛静止不动了。他眯起眼睛,集中精力,屏住呼吸,使劲儿想再……弯……那么一英寸……然后.他浑身的羽毛直竖,失去平衡.摔了下来。

    ④要知道.海鸥飞行时决不摇晃。决不失去平衡,盔空中失去平衡,对海鸥来说是极不光彩的事。

    ⑤但是乔纳森并不觉得丢脸.他再一次展开双翅,依旧颤抖着使劲弯曲——一点、一点地放慢速度,又一次失去平衡——他不是只平凡的鸟。

    ⑥大多数海鸥只求学会最简单的飞行本领…一如何从岸上飞出去觅食.再飞回来。对他们来说,重要的不是飞行,而是觅食。但对这只海鸥来说.重要的不是吃而是飞。乔纳森喜爱飞行胜于一切。

    ⑦他发现,像他这样的想法,在同类中是吃不开的。他那么整天独自练习.成百次地作低飞滑翔,连他的双亲都替他担心呢。

    ⑧他自己也不知道是什么原故.只要他保持离水面不到半翅的高度作低空飞行,他就能在空中停留较久,费劲较小。他滑翔下来并不像一般乌儿那样伸下双足溅落海中,而是蜷起双足紧贴着身体掠过海面,在水面留下长长一道波纹。他蜷起双足在沙滩上滑翔着陆,然后步测着沙滩上滑翔的距离,他的父母见了,着实为他担忧。

    ⑨“怎么啦,乔?怎么啦?”他妈妈问。“难道学其他海鸥的样儿这么难,乔?低飞是鹈鹕和信天翁的事,你学这干什么?你干吗不吃点儿?孩子,你都瘦得皮包骨头了!”

    ⑩“我倒不在乎瘦得皮包骨头.妈妈。我只是想知道我在空中能够做什么,不能够做什么。”

    ⑪“你瞧,乔纳森,”他父亲温和地说,“冬天快到了,船只就要少了,海面上的鱼也要钻到海底去了。你要是一定要学习,那就学学怎么觅食吧。飞行当然好,可你总不能拿滑翔当饭吃啊。别忘了,你飞行的目的就是为了吃。”

    ⑫乔纳森顺从地点点头。以后几天,他试着学其他海鸥的样儿;他作了认真的尝试,与鸥群一道围绕着码头和渔船嘎嘎叫着争食吃,扎到海里抢点儿面包片和烂鱼。但这样做他受不了。

    ⑬“这样太没意思了,”他心里想,一边故意把好不容易弄到的一条鲤鱼丢给一只追逐他的饥饿的老海鸥。

    ⑭不久,乔纳森又独自一个出去了。他飞到海上远处,饿着肚子学习,很是快乐。