单选题在力场的作用下，一质点沿圆周x2+y2=1逆时针运动一周所作的功为______A．B．C．D．

单选题One thing that Ismael Matos, 23, says he's learned on his job as a special agent with the Geek Squad, is that there are no stupid questions—not even when a customer asks where the power button is. The goal, Matos says, is first to "strip out the jargon. " You know what he means: those terms like gigs, Ram, and motherboard that civilians don't typically use in everyday conversations. Once he establishes a common language with the customer, Matos can focus on building a relationshipone that he hopes will pay off in the future. Matos knows that clients often don't have the same skills he does. His customers ask for help not only with setting up new computers and installing software, but also with digital cameras, wireless Internet access and even getting their iPod synced with iTunes. We live in the age of Web2.0, when Internet viewers are fast becoming content creators. We may log in to social networking sites such as Digg and Technorati, rate or even upload videos on YouTube, and contribute and edit information on collaborate websites known as wikis. But members of this expanding plugged-in population aren't necessarily up to speed with the language of the web—or understand the technical lingo of the sales or customer service people they're turning to for assistance. According to a "Cyber Stress" survey of 1,001 American broadband Internet service users, 46 percent said the typical tech support person uses an excess of technical language, and 61 percent said they would prefer a "computer therapist" who is compassionate and easy to talk to. A question that online public relations strategist Sally Falkow says she's hearing often these days is, "How do we cope with all this?" she tells people, "There's no way you can escape this. There's a big conversation going on," and it's important to learn how to be part of it. If you're at the earliest stage of the learning curve, start by reading technology-oriented columns and articles in magazines, newspapers and websites that are written for a general audience. Move up to more tech-oriented e-letters as you build a foundation. As you come across new tells, look them up online. Definitions at Wikipedia, the online encyclopedia written and edited by web users, may be helpful. You should also start associating with people who can help explain things. Look for groups that sponsor seminars or find a supportive online community. Don't be afraid to guide your helper. Explain what you need to know, ask him or her to slow down and speak in basic terms.

单选题将(ρcosθ，ρsinθ)dρ化成直角坐标下的累次积分，则I=

单选题极限______．

单选题n阶矩阵A与B有相同的特征向量是A与B相似的

单选题累次积分，可以写成A．．B．．C．．D．．

单选题设总体X服从标准正态分布，(X1，X2，…，Xn)为总体的简单样本

单选题A是3阶矩阵且AX=0有通解：k1α1+k2α2(k1，k2为任意常数)，又知Aα3=α3，P是三阶可逆矩阵，使，则P应该是______A．(α1，α2，α1+α3)B．(α2，α3，α1)C．(α1+α2，-α2，2α3)D．(α1+α2，α2+α3，α3)

单选题设有下列命题①数列{xn}收敛(即极限)，则xn有界．②数列极限其中l为某个确定的正整数．③数列④数列极限则以上命题中正确的个数是

单选题设随机事件A与B互不相容，则A．B．C．D．

单选题若对一切x∈(0，+∞)，函数f(x)的一、二阶导数均存在，且有f"(x)=0．则对任意正常数a，必有______．