使用显微镜时,转动下列哪种结构可以较大幅度的升降镜筒? A.遮光器 B.粗准焦螺

题型:选择题

问题:

使用显微镜时,转动下列哪种结构可以较大幅度的升降镜筒?

A.遮光器

B.粗准焦螺旋

C.转换器

D.细准焦螺旋

考点:显微镜的构造和使用
题型:选择题

下列关于阀门的选择说法错误的是( )。

A.一般压缩空气管道的阀门,主要是用于切断气流,可以采用可锻铸铁或铸铁制造
B.PN值不大于3MPa的氧气管道用阀门,可采用可锻铸铁、球墨铸铁或钢制阀门
C.真空度较高的真空系统可选用普遍截止阀或闸阀
D.一般取样、分析和仪表采用旋塞阀,截断、调节用截止阀或球阀

题型:选择题

下列城市化进程图分别表示的国家是( )

A.① 中国② 美国③ 英国④ 阿根廷

B.① 阿根廷② 美国③ 英国④ 中国

C.① 中国② 阿根廷③ 英国④ 美国

D.① 阿根廷② 英国③ 美国④ 中国

题型:选择题

下列关于子网掩码说法正确的是()

A.是用于确定IP地址中的网络ID和主机ID

B.用于将一个大的IP网络划分为若干小的子网络

C.子网掩码由32位(4个字节)表示

D.通过修改子网掩码可以改变网络ID和主机ID

题型:选择题

将加热杀死的S型肺炎链球菌细胞与R型活细胞一起注入小鼠体内,导致了小鼠死亡,这个实验结果是否能证明肺炎链球菌的遗传物质是DNA

题型:选择题

For the past two years in Silicon Valley, the centre of America’s technology industry, conference-goers have entertained themselves playing a guessing game: how many times will a speaker mention the phrase "long tail" It is usually a high number, thanks to the influence of the long-tail theory, which was first developed by Chris Anderson, the editor of Wired magazine, in an article in 2004. Though technologists and bloggers chuckle at how every business presentation now has to have its long-tail section, most are envious of Mr. Anderson, whose brainwave quickly became the most fashionable business idea around.

Whether a blockbuster film, a bestselling novel, or a chart-topping rap song, popular culture idolises the hit. Companies devote themselves to creating them because the cost of distribution and the limits of shelf space in physical shops mean that profitability depends on a high volume of sales. But around the beginning of this century a group of internet companies realised that with endless shelves and a national or even international audience online they could offer a huge range of products—and make money at the same time.

The niche, the obscure and the specialist, Mr. Anderson argues, will gain ground at the expense of the hit. As evidence, he points to a drop in the number of companies that traditionally calculate their revenue/sales ratio according to the 80/20 rule—where the top fifth of products contribute four-fifths of revenues. Ecast, a San Francisco digital jukebox company, found that 98% of its 10000 albums sold at least one track every three months. Expressed in the language of statistics, the experiences of Ecast and other companies such as Aragon, an online bookseller, suggest that products down in the long tail of a statistical distribution, added together, can be highly profitable. The internet helps people find their way to relatively obscure material with recommendations and reviews by other people, (and for those willing to have their artistic tastes predicted by a piece of software) computer programs which analyse past selections.

Long-tail enthusiasts argue that the whole of culture will benefit, not just commercial enterprises. Television, film and music are such bewitching media in their own right that many people are quite happy to watch and listen to what the mainstream provides. But if individuals have the opportunity to pick better, more ideally suited entertainment from a far wider selection, they will take it, according to the theory of the long tail. Some analysts reckon that entire populations might become happier and wiser once they have access to thousands of documentaries, independent films and subgenres of every kind of music, instead of being subjected to what Mr. Anderson calls the tyranny of lowest-common-denominator fare. That might be taking things a bit far. But the long tail is certainly one of the internet’s better gifts to humanity.

What’s the main idea of this passage()

A. Chris Anderson and his "Long-tail"

B. Prevalence of "Long-tail"

C. What is Long-tail

D. Long-tail and Internet

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