题干

如下图所示为科学家进行的蝾螈受精卵横缢实验,你认为该图最能说明

A:细胞质控制着细胞的代谢

B:细胞核控制着细胞的代谢

C:细胞质是细胞遗传特性的控制中心

D:细胞核控制细胞分裂、分化

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答案(点此获取答案解析)

D

同类题2

阅读下面的文章,完成后面小题。
打不碎的韧性陶瓷
①陶瓷,是用物理化学方法制造出来的最早的人造材料。一万多年以前,它的诞生使人类由旧石器时代进入了新石器时代。然而,陶瓷的脆性一直是它的致命弱点,如果能够克服这个弱点,再加上它原有的耐磨、耐高温、耐腐蚀等非常有价值的性能,那么陶瓷材料的前途将远远超过现代的优质合金。
②可喜的是,经过科学家的不懈努力,终于找到了克服陶瓷脆性的“药方”。
③首先,从改善内部结构着手。研究表明,在陶瓷的原料中添加少量的氧化钇、氧化镁、氧化钙等粉末,经高温炼制成氧化锆陶瓷后,其中的氧化锆便生成两种晶体:立方晶体和四方晶体。当陶瓷受到外力作用时,四方晶体便变成一种单斜晶体,体积迅速“膨胀”。由于晶体的体积急速增大,进而可阻止陶瓷中原先存在的细微裂纹的扩展,陶瓷就不会破裂了。有人在氧化铝坯料中加入二氧化锆,当加入量为基体体积的15%时,陶瓷的硬度可以提高3倍,有人称之为“陶瓷钢”。
④其次,将纤维均匀地分布于陶瓷的原料中,以提高陶瓷的强度和韧性,其原理与我们在石灰中加入纸筋相类似。这是因为,将纤维加入陶瓷原料中,具有以下三大作用:纤维不易拉断,在工作时可承担大部分外加负荷,减轻陶瓷的负担,使裂纹不易产生;纤维与陶瓷体结合在一起后,具有很大的摩擦力,于是,陶瓷的韧性可以大大增加;即使陶瓷出现了细微裂纹,纤维也能将它们紧紧拉住,使裂纹不至于进一步扩展开来。
⑤第三,还在改善陶瓷表面状态方面下功夫。一般来说,陶瓷的断裂大都从表面开始,因此,改善陶瓷表面状态就犹如为防止陶瓷的破损设下了一道屏障。具体方法为:通过化学和机械抛光技术消除陶瓷的表面缺陷;对氮化硅、碳化硅等非氧化物,只要通过控制表面氧化技术,就可消除表面缺陷或使裂纹尖端变钝;通过热处理也可达到表面强化或增韧的目的。
⑥经过特殊加工的陶瓷,再也不是那种碰不得、摔不起的“瓷娃娃”了,即使把它丢在水泥地上也毫无损伤。这种打不碎的陶瓷,目前已经广泛应用于一些高科技领域,如用来制造汽车、飞机、快艇的发动机和其他重要零部件。这类陶瓷材料无疑将有广阔的发展前景。

同类题4

阅读理解

    More companies and recyclers are taking steps to ensure that old electronic devices such as TVs and computers aren't sent to poor countries.

    The Basel Action Network, a Seattle-based non-profit that largely exposed the overseas discarding (丢弃) of US electronic waste, on Thursday launched a programme to use third-party employees to certify (证实) recyclers who don't export dangerous electronic waste.

    The so-called e-Steward recyclers will also agree not to discard the waste in US landfills and agree to meet other criteria. The certification is intended to provide companies and consumers with some assurance that the waste, which can include toxins (毒素) such as lead and mercury, is disposed of safely.

    The Government Accountability Office, in a 2008 report, declared that US electronic waste was often disposed of unsafely in such countries as India. There, workers recycle gold, silver and copper from the waste, often in open-air acid baths.

    The Basel Network also says it won assurances from 13 organizations, including Samsung, Bank of America, Wells Far-go, that they'll use e-Steward recyclers whenever possible. Wells Fargo had already been using recyclers who declared not to export. So far, Basel has certified three recyclers and seven sites.

    Before e-stewards, even, companies that wanted to avoid export of electronic waste had to “hope for the best”, when, they handed their waste to recyclers, says Robert Houghton, president of Ohio-based recycler Redemtech. It is an e-Steward that counts major companies among its customers. “Now, they can get some proof,” Houghton says.

    Basel's standards compete with another set launched in January. It was made by industry and backed by the Environmental Protection Agency.

    That standard, called R-2, doesn't ban the export of dangerous electronic waste but requires that it be handled safely. Instead of a ban, the Institute of Scrap Recycling Industries says, efforts should be made to help poor countries develop safe recycling.