题干

若  x<0,y>0时, x , x+y , y , x-y 中,最大的是(  )


A:x

B:x+y

C: x-y

D:y

上一题 下一题 0.0难度 选择题 更新时间:2019-09-03 05:58:40

答案(点此获取答案解析)

D

同类题4

阅读理解

    What are American high schools like? Well, I'm happy to tell you what I know.

    When I started school here, it had already been a week since the school opened. At this school, freshmen usually go on a trip for about three days at the beginning of school. Unfortunately I missed that wonderful trip, which would have been the best time to get to know my classmates. I was really sad. I wished I'd known about it earlier.

    Despite the disappointment, however, I gradually adapted to my new life and school.

    There is a space in the basement of the teaching building where students chat and meet each other. As we do not always have the same classrooms and classmates, the school wants us to get to know each other there. Students usually come to school early, sit in that space and have fun. Around the space, there are many lockers for students to leave their books in, so that students do not have to carry a heavy schoolbag everywhere.

    It really surprises me that we have almost no textbooks. We only have textbooks for World History and Algebra 2 and they are big and heavy, like bricks. For other classes, we only need binders (活页夹) with paper in them. Without textbooks, students learn things freely and actively. For example, my humanities teacher just teaches us what is in her mind at the time. We never know what we will learn.

    Another difference between American schools and Chinese schools is that American schools care about students' morality more than their academic studies. For example, if you do not finish your homework, you will just be asked to do it later, but if you cheat or lie, you will get a warning or even be kicked out.

    I think that most students here are good at schoolwork as well, but compared to Chinese students, they can make learning a more joyful experience. I think we should take the good points from our two different kinds of education to perfect our approach to studying.

同类题5

阅读下面的文字,完成后面小题。

大洋中脊的特殊地质结构

梁鑫峰

    ①自古以来,人类就知道陆地上有山脉;其实海洋里也有山脉,这些山脉被称作大洋中脊。

    ②人们意识到海底也有山脉,只不过___多年的时间。大洋中脊的发现,可以追溯到1872年英国“挑战者号”的全球调查。“挑战者”号上的科学家利用探测锤发现大西洋中部有一处高高隆起的地方。1925-1927年,在德国“流星”号考察期间,科学家利用声学回声测深法再次确认了这条山脉的存在。他们还发现这条位于大西洋底的山脉竟然通过好望角,延伸到了印度洋。

    ③在接下来的几十年间,印度洋中脊和东太平洋中脊也陆续被发现。二战后,船载声学设备的发展极大促进了人们对大洋中脊的研究。到20世纪50年代初期,地质学家已经知道地球上每个大洋的洋底都有山脉,并且这些山脉连成一个巨大的海底山脉系统。

    ④大洋中脊的发现使得人类对地球的认识也向前迈出了一大步。正是因为大洋中脊的发现,让魏格纳的大陆漂移学说开始得到重视,诞生了海底扩张学说,并进一步发展成现在被人们普遍接受的板块运动理论。

    ⑤大洋中脊既是海洋深处的巨大山脉,又是新的海洋洋壳生成的地方。大洋中脊的中轴线上坐落着众多的“火山口”。在那里,灼热的岩浆由地幔向上涌,逐渐冷却,结合周围已软化的岩石,形成新的洋壳。新生成的洋壳挤压大洋中脊两边已有的地壳,不断向外扩张,并最终在板块的交界边缘俯冲回地幔去。因此,洋壳在大洋中脊出生,在板块与板块的撞击中消亡。在过去的几十亿年里,大洋洋壳就这样的循环往复、生生不息。

    ⑥研究发现,大洋中脊相对其中轴线几乎是对称的。从中轴线往外,水深逐渐增加。比如大西洋中脊的中轴线处水深大约为2500米,但其外沿的水深则有5000多米。究其原因,中轴线处的洋壳刚刚形成,温度最高,因此其密度较小,所以也就升得最高。在几百万年的地质时间尺度上,中轴线处的洋壳往外移动,其温度逐渐降低,密度增加,随之也就慢慢沉下去了。于是,这些高大的海底山脉就形成了。有时,这些山脉还会露出海面形成岛屿,其中最著名的便是冰岛。

    ⑦虽然同为巨大的山脉,但是海底山脉和陆地山脉的生成机制不同。陆地山脉大多是由板块相互挤压形成的,大洋中脊则是新的海洋洋壳生成的地方。大多数大洋中脊的中轴线处存在一个被称为中央裂谷的特殊结构,形态类似山谷。大洋中脊就像是被从中轴线处劈开来一样。不过,不同的大洋中脊,其中央裂谷的形态也不同。比如,大西洋中脊的中央裂谷非常巨大,大概有25-30千米宽,且有1000-2000米深;而位于东太平洋中脊隆的中央裂谷只有不到1000米宽,深度也不到100米。这些差别主要是由于不同大洋中脊处新生成地壳的扩张速度不同。扩张速度快的(大于5厘米/年)大洋中脊会呈现出如东太平洋隆起样的结构,扩张速度慢的(小于5厘米/年)则更像大西洋中脊。

    ⑧大洋中脊以其特殊的地质结构吸引着众多的地质学家、海洋学家和生物学家。他们的发现和研究极大推动了人类对于地球和生命的认识进程。

(选自《中国国家地理杂志》,略有删改)