单选题When older people can no longer remember names at a cocktail party, they tend to think that their brainpower is declining. But a growing number of studies suggest that this assumption is often wrong. Instead, the research finds, the aging brain is simply taking in more data and trying to sift through a clutter of information, often to its long-term benefit. The studies are analyzed in a new edition of a neurology book, "Progress in Brain Research. " Some brains do deteriorate with age. Alzheimer's disease, for example, strikes 13 percent of Americans 65 and older. But for most aging adults, the authors say, much of what occurs is a gradually widening focus of attention that makes it more difficult to latch onto just one fact, like a name or a telephone number. Although that can be frustrating, it is often useful. "It may be that distractibility is not, in fact, a bad thing," said Shelley H. Carson, a psychology researcher at Harvard whose work was cited in the book. "It may increase the amount of information available to the conscious mind. " For example, in studies where subjects are asked to read passages that are interrupted with unexpected words or phrases, adults 60 and older work much more slowly than college students. Although the students plow through the texts at a consistent speed regardless of what the out-of-place words mean, older people slow down even more when the words are related to the topic at hand. That indicates that they are not just stumbling over the extra information, but are taking it in and processing it. When both groups were later asked questions for which the out-of-place words might be answers, the older adults responded much better than the students. "For the young people, it's as if the distraction never happened," said an author of the review, Lynn Hasher, a professor of psychology at the University of Toronto and a senior scientist at the Rotman Research Institute. "But for older adults, because they've retained all this extra data, they' re now suddenly the better problem solvers. They can transfer the information they've soaked up from one situation to another. " Such tendencies can yield big advantages in the real world, where it is not always clear what information is important, or will become important. A seemingly irrelevant point or suggestion in a memo can take on new meaning if the original plan changes. Or extra details that stole your attention, like others'yawning and fidgeting, may help you assess the speaker's real impact.

单选题设函数f(x)在(-∞，+∞)内连续，在(-∞，0)∪(0，+∞)内可导，函数y=y(x)的图像为则其导数的图像为______。A．B．C．D．

单选题下列反常积分发散的是{{U}}{{/U}}．

单选题给出以下三个二次型，①②③正定二次型的个数为()．

单选题设an＞0(n=1，2，…)且收敛，常数λ∈(0，)，则级数A.绝对收敛．B.条件收敛．C.发散．D.的敛散性与λ的取值有关．

单选题没X～t(2)，则服从的分布为______．

单选题设矩阵B的列向量线性无关，且BA=C，则( )

单选题It's generally been said that history is written by the winners. This was never more true than on March 12, when the Texas board of education voted 10-5 in favour of curriculum standards that would promote conservative takes on controversial issues in the pages of the state's textbooks. The changes, expected to win final approval in May, include an increased emphasis on and sympathetic treatment of such Republican standards as the National Rifle Association and the Moral Majority. They also boast the advantage of capitalism and the role of Christianity in the nation's founding. Even Thomas Jefferson's profile will be reduced; some board members were less than fond of his ideas about the division of church and state. This is not Texas' first such skirmish. Since the 1970s, the state has tried to drop books that were seen as too liberal or anti-Christian, to omit passages on the gay-rights movement and to tone down global-warming arguments. But the nation's battle over textbooks stretches back almost half a century earlier. In 1925, Tennessee's Butler Act (which was abolished in 1967) made it illegal to teach " any theory that denies the story of the Divine Creation of man as taught in the Bible. " The Scopes " monkey trial" famously followed. In 1974, a clash erupted in Kanawba County, West Virginia, over the controversial writings of such authors as George or well, Arthur Miller and Allen Ginsberg, Opposition was so heated that some schools were firebombed. As one of America's largest textbook buyers, the Longhorn State has a good deal of sway over what is sold to schools nationwide. And while Napoleon may have maintained that " history is the version of past events that people have decided to agree upon, " getting Texans to come together on the past may prove to be their Waterloo.

单选题是函数z=z(x，y)在点(x0，y0)处取得极值的______

单选题设事件A，B，C是一个完备事件组，即它们两两互不相容且其和为力，则下列结论中一定成立的是

单选题设D是以(1，1)，(-1，1)和(-1，-1)为顶点的三角形区域，D1是D在第一象限部分，则(xy+cosxsiny)dxdy=______。A．2cosxsinydxdyB．2xydxdyC．2(xy+cosxsiny)dxdyD．4cosxsinydxdy

单选题级数(α＞0，β＞0)的收敛性______

单选题已知函数f(x+y，x-y)=x2-y2对任何x与y成立，则等于A.2x-2y．B.2x+2y．C.x+y．D.x-y．