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The need for solar electricity is clear. It is safe, ecologically sound, efficient, continuously available, and is has no moving parts. The basic problem with the use of solar photovoltaic devices is economics, but until recently very little progress has been made toward the development of low-cost photovoltaic devices. The larger part of research funding has been devoted to study of single-crystal silicon solar cells, despite the evidence, including that of the leading manufacturers of crystalline silicon, that the technique holds little promise. The reason for this pattern is understandable and historical. Crystalline silicon is the active element in the Very successful semiconductor industry, and virtually all of the solid state devices contain silicon transistors and diodes. Crystalline silicon, however, is particularly unsuitable to terrestrial solar cells. Crystalline silicon solar cells work well and are successfully used in the space program, where cost is not an issue. While single-crystal silicon has been proven in extraterrestrial use with efficiencies as high as 18 percent, and other more expensive and scarce materials such as gallium arsenide can have even higher efficiencies, costs must be reduced by a factor of more than 100 to make them practical for commercial use. Beside the fact that the starting crystalline silicon is expensive, 95 percent of it is wasted and does not appear in the final device. Recently, there have been some imaginative attempts to make polycrystalline and ribbon silicon, which are lower in cost than high-quality single crystals. But to date the efficiencies of these apparently lower-cost arrays have been unacceptably small. Moreover, these materials are cheaper only because of the introduction of disordering in crystalline semiconductors, and disorder degrades the efficiency of crystalline solar cells. This dilemma can be avoided by preparing completely disordered or amorphous materials. Amorphous materials have disordered atomic structure as compared to crystalline materials. That is, they have only short-range order rather than the long-range periodicity of crystals. The advantages of amorphous solar cells are impressive. Whereas crystals can be grown as wafers about four inches in diameter, amorphous materials can be grown over large areas in a single process. Whereas crystalline silicon must be made 200 microns thick to absorb a sufficient, amount of sunlight for efficient energy conversion, only 1 micron of the proper amorphous materials is necessary. Crystalline silicon solar cells cost in excess of $100 per square foot, but amorphous films can be created at a cost of about 50 per square foot. Although many scientists were aware of the very low cost of amorphous solar cells, they felt that they could never be manufactured with the efficiencies necessary to contribute significantly to the demand for electric power. This was based on a misconception about the feature which determines efficiency. For example, it is not the conductivity of the material in the dark which is relevant, but only the photoconductivity, that is the conductivity in the presence of sunlight. Already, solar cells with efficiencies well above 6 percent have been developed using amorphous materials, and further research will doubtless find even less costly amorphous materials with higher efficiencies. The tone of the passage can best be described as ______.
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流通国债是指可以在流通市场上交易的国债。这种国债的特征是投资者可以自由认购、自由转让，通常不记名，转让价格取决于对该国债的供给与需求。( )
王某在某机票代理处购买四天后起飞的打折机票一张。第二天，由于特殊原因，王某向该机票代理处提出退票申请。机票代理处虽然同意退票，但告知王某打折机票的退票费需按机票原价的50%扣除，王某对此提出异议。日后，王某又与承运人某航空公司交涉。航空公司认为，本承运合同的当事人应是王某与机票代理处，自己不存在违约问题，尽管机票上印有退票条款，但退票费的收取应按航空行政管理部门有关内部文件的规定执行。王某认为，航空公司的行为违反了诚信原则，并且未尽告知义务，已构成违约。在协商未果的情况下，王某向法院提起诉讼，要求航空公司及代理处返还扣除的退票费。 相关背景资料： (1)机票上有关退票的规定：旅客在客票上列明的航班规定离站时间24小时以前要求退票的，退票手续费由承运人规定；在航班规定离站时间24小时以内至2小时以前要求退票的，收取客票价10%的退票费。 (2)航空行政管理部门有关“打折机票收取50%的退票费”的规定，未曾正式向外公告，只是在管理部门指定的航机读物，即行业报纸上以新闻形式出现。 (3)机票上有关退票的规定是执行航空旅客货物运输规则中的相关规定。 (4)航空行政管理部门有关打折机票退票费的文件实施时间，晚于航空旅客货物运输规则的实施时间。 (5)对机票上“退票手续费由承运人规定”的条文，航空公司没有证据证明原告已经知道。 根据法律规定，本案中( )。
平衡型基金一般将25%～50%的资产投资于债券及优先股，其余的投资于普通股。 ( )
Directions: There are 4 reading passages in this part. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C, and D. You should decide on the best choice and mark your answer on the ANSWER SHEET by blackening the corresponding letter in the brackets. Signs of deafness had given him great anxiety as early as 1798. For a long time he successfully concealed it from all but his most intimate friends, while he consulted physicians and quacks with eagerness. But neither quackery nor the best skill of his time availed him, and it has been pointed out that the root of the evil lay deeper than could have been supposed during his lifetime. Although his constitution was magnificently strong and his health was preserved by his passion for outdoor life, a post-mortem examination revealed a very complicated state of disorder, evidently dating from childhood (if not inherited) and aggravated by lack of care and good food. The touching document addressed to his brothers in 1802, and known as his "will" should be read in its entirety. No verbal quotation short of the whole will do justice to the overpowering outburst which runs in almost one long unpunctuated sentence through the whole tragedy of Beethoven’s life, as he knew it then and foresaw it. He reproaches men for their injustice in thinking and calling him pugnacious, stubborn, and misanthropical when they do not know that for six years he has suffered from an incurable condition aggravted by incompetent doctors. He dwells upon his delight in human society from which he has had so early to isolate himself, but the thought of which now fills him with dread as it makes him realize his loss, not only in music but in all finer interchange of ideas, and terrifies him lest the cause of his distresses should appear. He declares that, when those near him had heard a flute or a singing shepherd while he heard nothing, he was only prevented from taking his life by the thought of his art, but it seemed impossible for him to leave the world until he had brought out all that he felt to be in his power. He requests that after his death his present doctor, if surviving, shall be asked to describe his illness and to append it to this document in order that at least then the world may be as far as possible reconciled with him. He leaves his brothers property, such as it is, and in terms not less touching, if more conventional than the rest of the document, he declares that his experience shows that only virtue has preserved his life and his courage through all his misery. During the last twelve years of his life, his nephew was the cause of most of his anxiety and distress. His brother, Kaspar Karl, had often given him trouble--for example, by obtaining and publishing some of Beethoven’s early indiscretions, such as the trio variations, op. 44, the sonatas, op. 49, and other trifles. In 1815, after Beethoven had quarreled with his oldest friend, Stephan Breuning, for warning him against trusting his brother in money matters, Kaspar died, leaving a widow of whom Beethoven strongly disapproved, and a son, nine years old, for the guardianship of whom Beethoven fought the widow through all the law courts. The boy turned out utterly unworthy of his uncle’s persistent devotion and gave him every cause for anxiety. He failed in all his examinations, including an attempt to learn some trade in all his ecaminations, including an attempt to learn some trade in the polytechnic school, whereupon he fell into the hands of the police for attempting suicide, and after being expelled from Vienna, joined the army. Beethoven’s utterly simple nature could neither educate nor understand a human being who was not possessed by the wish to do his best. His nature was passionately affectionate, and he had suffered all his life from the want of a natural outlet for it. He had often been deeply in love and made no secret of it. But Robert Browning had not a more intense dislike of "the artistic temperament" in morals, and though Beethoven’s attachments were almost hopelessly above him in rank, there is not one that was not honorable and respected by society as showing the truthfulness and self-control of a great man. Beethoven’s orthodoxy in such matters has provoked the smiles of Philistines, especially when it showed itself in his objections to Mozart’s Don Giovanni and the grounds for selecting the subject of Fidelio for his own opera. The last thing that Philistines will ever understand is that genius is far too independent of convention to abuse it, and Beethoven’s life, with all its mistakes, its grotesqueness, and its pathos, is as far beyond the shafts of Philistine wit as his art. Opus 44 and Opus 49 are called "indiscretions" because they were ______.
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