单选题蛋白质含氮量平均约为

单选题下列机制中，最主要参与解氨毒作用的是（ ）

单选题 关于嘧啶二聚体形成下列说法错误的是 A、属于DNA链内交联 B、形式有TT、CT、CC C、属于DNA链间交联 D、可由光复活酶修复 E、紫外线照射可引起此种损伤

单选题线粒体氧化磷酸化解偶联意味着

单选题嘌呤环的合成中向嘌呤环只提供一个碳原子的化合物是

单选题关于脆性X综合征的叙述，不正确的是

单选题血液中结合胆红素的蛋内质是（ ）

单选题下列辅酶中的哪个不是来自于B族维生素

单选题酶与一般催化剂的区别是

单选题关于脂肪酸β-氧化的叙述，错误的是

单选题下列关于酶化学修饰调节的叙述，错误的是

单选题下列关于物质在体内氧化和体外燃烧的特点，正确的是

单选题不属于高能磷酸化合物的是

单选题谷氨酸脱氨基作用的产物是

单选题关于转氨基作用的叙述，错误的是

单选题痛风症时，沉积于关节、肾等处的成分是

单选题有机磷农药中毒主要是它们能特异地和酶活性中心哪种基团结合

单选题下列能自由透过线粒体膜的物质是

单选题下列哪项属组成性基因表达

单选题In the two decades between 1910 and 1930, over ten percent of the Black population of the United States left the South, where the majority of the Black population had been located, and migrated to northern states, with the largest number moving, it is claimed, between 1016 and 1918. It has been frequently assumed, but not proved, that most of the migrants in what has come to be called the Great Migration came from rural areas and were motivated by two concurrent factors: the collapse of cotton industry following boll weevil infestation, which began in 1898, and increased demand in the North for labor following the cessation of European immigration caused by the outbreak of the First World War in 1914. This assumption has led to the conclusion that the migrants' subsequent lack of economic mobility in the North is tied to rural background, a background that implies unfamiliarity with urban living and a lack of industrial skills. But the question of who actually left the South has never been investigated in detail. Although numerous investigations document a flight from rural southern areas to southern cities prior to the Great Migration, no one has considered whether the same migrants then moved on to northern cities. In 1910 over 600,000 Black workers, or ten percent of the Black work force reported themselves to be engaged in "manufacturing and mechanical pursuits", the federal census category roughly including the entire industrial sector. The Great Migration could easily have been made up entirely of this group and their families. It is perhaps surprising to argue that an employed population could be tempted to move, but an explanation lies in the labor conditions then prevalent in the South. About thirty-five percent of the urban Black population in the South was engaged in skilled trades. Some were from the old artisan class of slavery--blacksmiths, masons, carpenters--which had a monopoly of certain trades, but they were gradually being pushed out by competition, mechanization, and obsolescence. The remaining sixty-five percent, more recently urbanized, worked in newly developed industries--tobacco, lumber, coal and iron manufacture, and railroads. Wages in the South, however, were low, and Black workers were aware, through labor recruiters and the Black press, that they could earn more even as unskilled workers in the North than they could as artisans in the South. After the boll weevil infestation, urban Black workers faced competition from the continuing influx of both Black and White rural workers, who Were driven to undercut the wages formerly paid for industrial jobs. Thus, a move north would be seen as advantageous to a group that was already urbanized and steadily employed, and the easy conclusion tying their subsequent economic problems in the North to their rural backgrounds comes into question. Notes: boll weevil infestation 棉铃虫蔓延。cessation 中止，停止。mason 泥瓦匠。recruiter 招募者。influx流入，涌入。