如图所示,使用中属于费力杠杆的是( ) A.核桃夹 B.起子 C.镊子 D.羊角

题型:选择题

问题:

如图所示,使用中属于费力杠杆的是(  )

A.核桃夹

B.起子

C.镊子

D.羊角锤

考点:杠杆及其五要素
题型:选择题

食品用塑料包装材料的卫生安全性主要包括().

A、无毒性

B、抗生物侵入性

C、耐腐蚀性

D、防有害物质渗透性

题型:选择题

设真空中点电荷+q1和点电荷+q2相距2a,且q2=2q1,以+q1为中心,a为半径形成封闭球面,则通过该球面的电通量为()。

A.3q1

B.2q1

C.q1

D.0

题型:选择题

在哪几种情形下烟草专卖行政管理部门应当立案查处?

题型:选择题

At some point during their education, biology students are told about a conversation in a pub that took place over 50 years ago. J. B. S. Haldane, a British geneticist, was asked whether he would lay down his life for his country. After doing a quick calculation on the back of a napkin, he said he would do so for two brothers or eight cousins. In other words, he would die to protect the equivalent of his genetic contribution to the next generation.

The theory of kin selection--the idea that animals can pass on their genes by helping their close relatives--is biology’s explanation for seemingly altruistic acts. An individual carrying genes that promote altruism might be expected to die younger than one with "selfish" genes, and thus to have a reduced contribution to the next generation’s genetic pool. But if the same individual acts altruistically to protect its relatives, genes for altruistic behavior might nevertheless propagate.

Acts of apparent altruism to non-relatives can also be explained away, in what has become a cottage industry within biology. An animal might care for the offspring of another that it is unrelated to because it hopes to obtain the same benefits for itself later on (a phenomenon known as reciprocal altruism). The hunter who generously shares his spoils with others may be doing so in order to signal his superior status to females, and ultimately boost his breeding success. These apparently selfless acts are therefore disguised acts of self interest.All of these examples fit economists’ arguments that Homo sapiens is also Homo economicus--maximizing something that economists call utility, and biologists fitness. But there is a residuum of human activity that defies such explanations: people contribute to charities for the homeless, return lost wallets, do voluntary work and tip waiters in restaurants to which they do not plan to return. Both economic rationalism and natural selection offer few explanations for such random acts of kindness. Nor can they easily explain the opposite: spiteful behavior, when someone harms his own interest in order to damage that of another. But people are now trying to find answers.

When a new phenomenon is recognized by science, a name always helps. In a paper in Human Nature, Dr Fehr and his colleagues argue for a behavioral propensity they call "p reciprocity". This name is intended to distinguish it from reciprocal altruism. According to Dr Fehr, a person is a p reciprocator if he is willing to sacrifice resources to be kind to those who are being kind, and to punish those who are being unkind. Significantly, p reciprocators will behave this way even if doing so provides no prospect of material rewards in the future.

The story of J. B. S. Haldane is mentioned in the text()

A.to honor his unusual altruistic acts

B. to show how he contributed to his offspring

C. to introduce the topic of human altruism

D. to give an episode of his calculation abilities

题型:选择题

无轨自行式起重机包括()。

A、履带式起重机

B、汽车式起重机

C、轮胎式起重机

D、塔吊

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