单选题The livelihood of each species in the vast and intricate assemblage of living things depends on the existence of other organisms. This interdependence is sometimes subtle, sometimes obvious. Perhaps the most straightforward dependence of one species on another occurs with parasites, organisms that live on or in other living things and derive nutrients directly from them. The parasitic way of life is widespread. A multitude of microorganisms (including viruses and bacteria) and an army of invertebrates——or creatures lacking a spinal column (including crustaceans, insects, and many different types of worms)—— make their livings directly at the expense of other creatures. In the face of this onslaught, living things have evolved a variety of defense mechanisms for protecting their bodies from invasion by other organisms. Certain fungi and even some kinds of bacteria secrete substances known as antibiotics into their environment. These substances are capable of killing or inhibiting the growth of various kinds of bacteria that also occupy the area, thereby eliminating or reducing the competition for nutrients. The same principle is used in defense against invaders in other groups of organisms. For example, when attacked by diseasecausing fungi or bacteria, many kinds of plants produce chemicals that help to ward off the invaders. Members of the animal kingdom have developed a variety of defense mechanisms for dealing with parasites. Although these mechanisms vary considerably, all major groups of animals are capable of detecting and reacting to the presence of "foreign" cells. In fact, throughout the animal kingdom, from sponges to certain types of worms, shellfish, and all vertebrates (creatures possessing a spinal column), there is evidence that transplants of cells or fragments of tissues into an animal are accepted only if they come from genetically compatible or closely related individuals. The ability to distinguish between "self" and "nonself", while present in all animals, is roost efficient among vertebrates, which have developed an immune system as their defense mechanism. The immune system recognizes and takes action against foreign invaders and transplanted tissues that are treated as foreign cells.

单选题 Whenever a rattlesnake is agitated, it begins to move its tail and make a rattling noise.

单选题One of the saddest things about the period in which we live is the growing estrangement between America and Europe. This may be a surprising discovery to those who are over impressed by the speed with which turbojets can hop from New York to Paris. But to anyone who is aware of what America once meant to English libertarian poets and philosophers, to the young Ibsen bitterly excoriating European royalty for the murder of Lincoln, to Italian novelists and poets translating the nineteenth century American classics as a demonstration against Fascism, there is something particularly disquieting in the way that the European Left, historically "pro-American" because it identified America with expansive democracy, now punishes America with Europe's lack of hope in the future. Although America has obviously not fulfilled the visionary hope entertained for it in the romantic heyday, Americans have, until recently, thought of themselves as an idea, a "proposition" (in Lincoln's word) set up for the enlightenment and the improvement of mankind. Officially, we live by our original principles; we insist on this boastfully and even inhumanly. And it is precisely this steadfastness to principle that irks Europeans who under so many pressures have had to shift and to change, to compromise and to retreat. Historically, the obstinacy of America's faith in "principles" has been staggering—the sacrament of the Constitution, the legacy of the Founding Fathers, the Moral Tightness of all our policies, the invincibility of our faith in the equality and perfectibility of man. From the European point of view, there is something impossibly romantic, visionary, and finally outrageous about an attachment to political formulas that arose even before a European revolutionary democracy was born of the French Revolution, and that have survived all the socialist Utopias and internationals. Americans honestly insist on the equality of men even when they deny this equality in practice; they hold fast to romantic doctrines of perfectibility even when such doctrines contradict their actual or their formal faith—whether it be as scientists or as orthodox Christians. It is a fact that while Americans as a people are notoriously empirical, pragmatic, and unintellectual, they live their lives against a background of unalterable national shibboleths. The same abundance of theory that allowed Walt Whitman to fill out his poetry with philosophical road signs of American optimism allows a president to make pious references to God as an American tradition—references which, despite their somewhat mechanical quality, are not only sincere but which, to most Americans, express the reality of America.

单选题Military orders are ______ and cannot be disobeyed.

单选题Tony was in plain clothes, watching for a______character at London Airport all night.

单选题With most online recruitment services, jobseekers must choose their words carefully, ______ the search engine will never make the correct match.

单选题An investigation that is ______ can occasionally yield new facts, even notable ones, but typically the appearance of such facts is the result of a search in a definite direction. A. timely B. unguided C. consistent D. subjective

单选题When the fire broke out in the building, the people {{U}}lost their heads{{/U}} and ran into the elevator.

单选题He will agree to do what you require ______ him. A. of B. from C. to D. for

单选题It is doubtful whether anyone can be a truly______ observer of events.(2003年清华大学考博试题)

单选题As the ______ to the general strike the management promised to increase the workers' payment. A. succession B. concession C. permission D. pledge

单选题A good way to ______ a language is to live in the native culture with the native speakers. A. require B. inquire C. acquire D. enquire

单选题If something very substantial is not done next month, he cannot ______ his office.

单选题For years now, the people of that faraway country have been cruelly ______ by a dictator.

单选题The appeal for funds did not ______ much of respond

单选题It is hard to predict how science is going to turn out, and if it is really good science it is impossible to predict. If the things to be found are actually new, they are by definition unknown in advance. You cannot make choice in that matter. You either have science or you don"t have, and if you have it you are obliged to accept the surprising and disturbing pieces of information, along with the neat and promptly useful bits. The only solid piece of scientific truth about which I feel totally confident is that we are profoundly ignorant about nature. I regard this as the maj or discovery of the past hundred years of biology. It is, in its way, an illuminating piece of news. It would have amazed the brightest minds of the 18th century Enlightenment to be told by any of us how little we known and how bewildering seems the way ahead. It is this sudden confrontation with the depth and scope of ignorance that represents the most significant contribution of the 20th century science to the human intellect. In earlier times, we either pretended to understand how things worked or ignored the problem, or simply make up stories to fill the gaps. Now that we have begun exploring in eamest, we are getting glimpses of how huge the questions are, and how far they are from being answered. Because of this, we are depressed. It is not so bad being ignorant if you are totally ignorant. The hard thing is knowing in some detail the reality of ignorance, the worst spots and here and there the not-so-bad spots, but no true light at the end of the tunnel nor even any tunnels that can yet be trusted. But we are making a beginning, and there ought to be some satisfaction. There are probably no questions we can think up that can"t be answered, sooner or later, including even the matter of consciousness. To be sure, there may well be questions we can"t think up ever, and therefore limits to the reach of human intellect, but that is another matter. Within our limits, we should be able to work our way through to all our answers, if we keep at it long enough, and pay attention.

单选题If you want to get into that tunnel, you have to ______ away all the rocks. [A] repel [B] haul [C] transfer [D] dispose

单选题In the same way, children learning to do all the other things A they learn to do without being taught—to walk, run, climb, whistle, ride a bicycle—compare their own performances B with that of C more skilled people, and slowly make D the needed changes .