下列物质中,含有共价键的离子化合物是[ ] A.Na2O2 B.MgCl2 C.

题型:选择题

问题:

下列物质中,含有共价键的离子化合物是[ ]

A.Na2O2

B.MgCl2

C.NH3

D.KCl

考点:离子化合物共价键
题型:选择题
已知等差数列{an}的前n项和为Sn,若a4=8,a5=10,则S8=(  )
A.18B.36C.54D.72
题型:选择题

下列关于噪声环境影响评价的基本内容说法正确的是( )。

A.噪声环境影响评价的基本内容包括六个方面

B.评价分析项目建设前环境噪声现状

C.根据噪声预测结果和噪声环境评价标准,评述建设项目施工、运行阶段噪声的影响程度、影响范围及超标情况

D.仅分析受噪声影响的超标人口的分布

题型:选择题

心脏起搏器

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Looking back on my childhood, I am convinced (1) naturalists are born and not made. Although we were all brought (2) in the same way, my brothers and sisters soon gave up their pressed flowers and insects. (3) them, I had no ear (4) music and languages, I was not an early reader and I could not do mental arithmetic.Before World War I we (5) our summer holidays in Hungary. I have only the memory of the house we lived (6) , of my room and my toys. I can hardly (7) clearly the large family of grandparents, aunts, uncles and cousins who gathered (8) door. But I do have a clear memory of the dogs, the farm (9) , the local birds and, (10) all, the insects.I am a (11) , not a scientist. I have a p (12) of the natural world, and my enthusiasm has led me into varied investigation. I love discussing my favorite topics and enjoy (13)  the midnight oil  (14)  reading about other people’s observations and discoveries. Then  (15)  happens that brings these observations together  (16)  my conscious mind. Suddenly you fancy (想象), you see the answer  (17)  the riddle, and books, of which some might honor with the title of scientific research.But curiosity, a keen eye, a good memory and enjoyment of the animal and plant world do not make a scientist; one of the outstanding and essential  (18)  required is self-discipline, a quality I lack. A scientist requires not only  (19)  but also hard training, determination and a goal. A scientist, up to a point, can be made. A naturalist is  (20) . If you can combine the two, you get the best of both worlds.

 

第(15)空应选择()

A.somewhat

B.someone

C.somewhere

D.something

题型:选择题

内燃式发动机每次启动时间不得超过()秒。

A、5

B、10

C、15

D、20

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