题干

设x,y∈R,则(x﹣y)x4<0是x<y的(   )

A:充分不必要条件

B:必要不充分条件

C:充要条件

D:既不充分也不必要条件

上一题 下一题 0.0难度 选择题 更新时间:2019-05-12 10:08:41

答案(点此获取答案解析)

A

同类题3

阅读下文,回答问题

动物的奇特语言

    ①人类的信息交流离不开语言。那么,动物有没有“语言”呢?常言说:“人有人言,兽有兽语。”所谓动物的“语言”,一般是指同一种动物之间为了种群的生存和发展传递信息的动作、气味和声音。

    ②动物的动作是一种常见的“语言”。例如,鹿科动物的小尾巴反面是白色的,当它们遇到危险时,就会把尾巴撅起,露出反面的白色,像摇动着白色的“信号”,在绿色丛林地带非常显眼,可以告诉自己的同类:跟我来,快逃跑。工蜂发现蜜源后,会通过圆形舞和“8”字摆尾舞把蜜源的距离和方向告诉自己的同伴。

    ③气味是一种特殊的“语言”。昆虫体表的腺体能分泌信息激素,这是一类挥发性的有独特气味的化学物质。在昆虫释放的信息激素中,最普遍、最灵敏、最专一的,是吸引异性的“性引诱素”。例如,一种雌蛾在交配产卵前,能分泌只有0.1微克的“性引诱素”,但雄蛾依靠头部的触觉,甚至在1公里外都能收到这种信号,几乎只要一个气味分子作用于雄蛾的触角,就足以引诱雄蛾与雌蛾“约会”。蚁、蜂、蚜虫等受到伤害或惊扰时,会放出一种“警戒素”,以告诉同类赶快逃避或奋起自卫。

    ④高等脊椎动物也有气味“语言”。例如,人们在小狗身上抹上母猪的尿液,母猪凭着这种体味信号,会把小狗当作自己的孩子进行哺乳。雄羚羊求偶时,也要靠嗅觉来识别雌羚羊。

    ⑤当然,动物的声音更是一种重要的“语言”。根据动物学家研究,猪有23种声音信号,狐狸有36种声音信号,而阿拉伯狒狒发出的声音信号竟然不少于40种。在模拟声音能力方面,一种叫库科尼亚的鹦鹉会“说”上百个单词和几十句话。

同类题5

阅读理解

    A 15-year-old student who invented a flashlight(手电筒) getting power from the holder's body heat is going home today from California with a big prize and a chance to do further research.

    Ann Makosinski was the only Canadian among the four winners at Google's international science competition. Thousands of young scientists from around the world took part in the competition.

    Winning the science and technology competition was "a surprise". Ann said, "I think it will have a great influence on my future."

    Ann thanked her family for encouraging her interest in science and said that her first toy was a box of transistors(晶体管).

    Ann's prize includes $ 25,000 and a "once in a-lifetime expericnce" from Google for her Hollow Flashlight, which has no moving parts or batteries.

    The idea for the invention came from seeing unwanted batteries and her friend's experience. When Ann visited a friend in the Philippines, she saw the friend couldn't study when it became dark because there was no electricity or light. She saw the need for a flashlight that has no batteries - Hollow Flashlight.

    In her project, Ann wrote "I made two flashlights that do not use any batteries or harmful materials. They do not create any noise and will always work. The flashlight needs at least a 5℃ temperature difference between the holder's body and the environment around to produce light."

    A video of Ann explaining how she creatcd the flashlight has been watched more than 1.4 million times on the Internet.

    Though Ann was successful, she has not made a decision about her career path. Ann hopes that she can find a way to join her love of film and science together.

    The four winners were chosen from 15 final competitors from eight countries. The competition attracted thousands of students in 120 countries.