题干

下列有关某人体内神经细胞、内分泌腺细胞和免疫细胞的叙述,正确的是(    )

A:上述细胞都具有成形的细胞核,都能在显微镜下观察到染色体

B:上述细胞的形态、功能各不同,这是因为不同细胞内所含基因不同

C:所有的免疫细胞都有识别抗体的功能,神经细胞和内分泌腺细胞也有相应的识别功能

D:神经细胞分泌递质与内分泌腺细胞分泌蛋白质类激素的方式相同

上一题 下一题 0.0难度 选择题 更新时间:2013-03-04 12:48:51

答案(点此获取答案解析)

D

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阅读短文,回答问题:

半导体的应用

半导体指常温下导电性能介于导体与绝缘体之间的材料.半导体是指一种导电性可受控制,范围可从绝缘体至导体之间的材料.

发光二极管(LED)是半导体二极管的一种,具有单向导电性.当二极管两端外加正向电压时,如果正向电压很小,正向电流也几乎为零,当正向电压大于某个值以后,二极管正向导通,电流随电压增大而迅速上升.当二极管两端外加反向电压不超过一定范围时,通过二极管的电流几乎为零,二极管处于截止状态.外加反向电压超过某一数值时,反向电流会突然增大,这种现象称为电击穿.因而使用时应避免二极管外加的反向电压过高.如图甲所示为某发光二极管中的电流随它两端电压变化的曲线.

PTC是一种新型的半导体陶瓷材料,它以钛酸钡为主,渗入多种物质后加工而成,目前家用的陶瓷暖风器、陶瓷电热水壶等就是用这种材料做成的.PTC有一个根据需要设定的温度,低于这个温度时,其电阻随温度的升高而减小,高于这个温度时,电阻值则随温度的升高而增大,我们把这个设定的温度叫“居里点温度”,用PTC材料制成的电热器具有发热、控温双重功能,应用十分广泛.

家用固体电热灭蚊器也使用了PTC陶瓷电热元件,该元件电阻与温度的关系如图所示.

请根据上述材料,回答下列问题:

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阅读理解

    The first ancient Olympic Games for which we still have written records were held in 776 BC. Coroebus won the only event at the Olympics. This made him the very first Olympic champion in history. Then they grew and continued to be played every four years. In 393 AD, the Roman emperor Theodosius Ⅰ, a Christian, announced to end the Games because of their pagan (异教的) influences.

    About 1,500 years later, a young Frenchman named Pierre de Coubertin began their revival (恢复). In 1892 Coubertin first brought forward his idea to revive the Olympic Games but failed.

    Two years later, Coubertin organized a meeting with 79 delegates (代表) who represented nine countries. At this meeting he got what he wanted. All the delegates at the conference voted for the Olympic Games. They also decided to have Coubertin set up an international committee to organize the Games. This committee became the International Olympic Committee (IOC) and Demetrious Vikelas from Greece was selected to be its first president. Athens was chosen for the revival of the Olympic Games and the planning was begun.

    The very first modern Olympic Games opened in the first week of April, 1896. Since the Greek government had been unable to afford a stadium, a rich Greek architect, Georgios Averoff, donated one million drachmas (over $100,000) to repair the Panathenaic Stadium, originally built in 330 BC.

    Since the Games were not well publicized internationally, contestants (选手) were not nationally chosen but rather came individually and at their own expense. Some contestants were tourists who happened to be in the area during the Games.