单选题设二元函数f(x，y)在点(x0，y0)处的两个偏导数f'x(x0，y0)与f'y(x0，y0)都存在，则(A)极限与极限都存在．(B)极限存在，但函数f(x，y)在点(x0，y0)处不一定连续．(C)函数f(x，y)在点(x0，y0)处连续．(D)函数f(x，y)在点(x0，y0)处可微，且全微分df|(x0,y0)=f'x(x0，y0)dx+f'y(x0，y0)dy．

单选题设A是m×n矩阵，r

单选题设则______．A．B=P1AP2B．B=P2AP1C．D．

单选题设an≠0(n=1，2，…)且()

单选题X1，X2，…，Xn(n＞1)是来自泊松总体P(λ)的简单随机样本，有则下列关系正确的是______A．B．C．D．不确定．

单选题设f(x，y)为连续函数，则使成立的充分条件是()

单选题设f(x)在x=n处可导，则|f(x)|在x=a处不可导的充要条件是 ( )．

单选题设A为n阶方阵，齐次线性方程组Ax=0有两个线性无关的解，A*是A的伴随矩阵，则有______． A.A*x=0的解均为Ax=0的解 B.Ax=0的解均为A*x=0的解 C.Ax=0与A*x=0无非零公共解 D.Ax=0与A*x=0恰好有—个非零公共解

单选题It is no longer just dirty blue-collar jobs in manufacturing that are being sucked offshore but also white-collar service jobs, which used to be considered safe from foreign competition. Telecoms charges have tumbled, allowing workers in far-flung locations to be connected cheaply to customers in the developed world. This has made it possible to offshore services that were once non-tradable. Morgan Stanley's Mr. Roach has been drawing attention to the fact that the "global labour arbitrage" is moving rapidly to the better kinds of jobs. It is no longer just basic data processing and call centres that are being outsourced to low-wage countries, but also software programming, medical diagnostics, engineering design, law, accounting, finance and business consulting. These can now be delivered electronically from anywhere in the world, exposing skilled white-collar workers to greater competition. The standard retort to such arguments is that outsourcing abroad is too small to matter much. So far fewer than lm American service-sector jobs have been lost to off-shoring. Forrester Research forecasts that by 2015 a total of 3.4m jobs in services will have moved abroad, but that is tiny compared with the 30m jobs destroyed and created in America every year. The trouble is that such studies allow only for the sorts of jobs that are already being off-shored, when in reality the proportion of jobs that can be moved will rise as IT advances and education improves in emerging economies. Alan Blinder, an economist at Princeton University, believes that most economists are underestimating the disruptive effects of off-shoring, and that in future two to three times as many service jobs will be susceptible to off-shoring as in manufacturing. This would imply that at least 30% of all jobs might be at risk. In practice the number of jobs off-shored to China or India is likely to remain fairly modest. Even so, the mere threat that they could be shifted will depress wages. Moreover, says Mr. Blinder, education offers no protection. Highly skilled accountants, radiologists or computer programmers now have to compete with electronically delivered competition from abroad, whereas humble taxi drivers, janitors and crane operators remain safe from off-shoring. This may help to explain why the real median wage of American graduates has fallen by 6% since 9000, a bigger decline than in average wages. In the 1980s and early 1990s, the pay gap between low-paid, low-skilled workers and high-paid, high-skilled Workers widened significantly. But since then, according to a study by David Autor, Lawrence Katz and Melissa Kearney, in America, Britain and Germany workers at the bottom as well as at the top have done better than those in the middle-income group. Office cleaning cannot be done by workers in India. It is the easily standardised skilled jobs in the middle, such as accounting, that are now being squeezed hardest. A study by Bradford Jensen and Lori Kletzer, at the Institute for International Economics in Washington D. C., confirms that workers in tradable services that are exposed to foreign competition tend to be more skilled than workers in non-tradable services and tradable manufacturing industries.

单选题设对n=1，2，3，…总有不等式an≤bn≤cn，则A．若与都收敛，则必有收敛．B．若与都发散，则必有发散．C．D．仅由级数与的敛散性未必能判定级数的敛散性．

单选题Some would consider that an infringement of good manners whereas others would not. A. an example B. a violation C. a problem D. an indication