题干

建立完整的电磁场理论并首先预言电磁波存在的科学家是 (      )

A:法拉第

B:奥斯特

C:麦克斯韦

D:赫兹

上一题 下一题 0.0难度 选择题 更新时间:2016-01-05 02:53:46

答案(点此获取答案解析)

C

同类题1

现代文阅读
抗生素之殇
    ①1928年,英国微生物学家亚历山大·费莱明首次从青霉菌中发现了青霉素,从此人类进入了抗生素的黄金时代。第二次世界大战中,青霉素作为一线药用抗生素拯救了成千上万人的性命,价格曾一度比黄金还要昂贵。但在60多年后,很多抗生素失却了当年的雄风,成了医药学家的心头之痛。
    ②除了抗生素天然存在的毒副作用,抗生素耐药性问题是当今社会面临的重大问题之一。微生物适应环境的本领很强,当环境剧变时,它们不是束手待毙,而是用基因突变与环境抗争。生物与环境是统一的整体,成天生活在抗生素环境中的细菌,大部分不能存活,但总有漏网之鱼。侥幸存活的细菌在环境逼迫下会发生基因突变,一旦突变出抗药基因,就会绝处逢生。
    ③在大自然漫长的进化过程中,有“矛”就必有“盾”,抗生素固然是大自然产生的有效武器,但是大自然也必然保留着相应的破解方法。研究人员目前已经发现了数十类病原体具有耐受抗生素的抗性调控机制,这些病原体能够有效应对环境中出现的不同类型的抗生素。分裂生长速度越快的病原体,其耐药性产生的速度也就越快。
    ④据统计,人类使用抗生素之前和使用初期,世界范围内的耐药菌出现速度远慢于当今,随着不同种类抗生素的不断开发,临床上耐药菌开始呈指数级地出现。近年,多重耐药“超级细菌”的出现曾一度引起社会的恐慌,而目前新型抗生素的研发速度远慢于临床中耐药菌的产生速度。
    ⑤人们对于抗生素的滥用,正是在对耐药菌进行着无形的“人工筛选”。而如今世界范围内发达的交通运输使得这些人工筛选得到的耐药菌可以快速传播。即使你得了病坚持不用抗生素,体内确实没有筛选出大量的耐药菌,但是仍然有可能躺枪。
    ⑥虽然抗生素滥用的后果十分可怕,但是也不必杞人忧天和坐以待毙。首先,应该从每个人做起,不要感冒发烧后就自己给自己开药,应该遵循医嘱,科学合理用药。其次,应该呼吁医药和食品行业不要滥用抗生素,毫无节制地滥用抗生素无异于自掘坟墓。医药学家也正在与耐药菌赛跑,一方面改变药物的分子机构,以对付病原菌突变合成的酶;另一方面从世界各个生态环境中筛选新的抗生菌种。无论是赤日炎炎的热带,还是寒冷的极地;无论是大洋深处的喷口,还是地下的岩层,都有抗生素专家探求的目光。
(本文有删改)

同类题4

根据短文内容,选择最佳答案。

    China dropped its decades-long, one-child policy last year to allow each family to have two children. This change has put 270 million married women of childbearing age in the position of choosing between family and work. The employers also face big challenges as more female workers will have two maternity leaves(产假) for a total of seven to eight months.

    In a survey published by classified advertising website Ganji.com, career women who might be considering having a second child were asked what kinds of pressure they might expect. More than 76 percent of the women who were questioned mentioned concerns about the financial burden of raising two children, while more than 71 percent said it would be difficult to balance career and family. In addition, nearly 56 percent said that having a second child would definitely have a negative effect on their career.

    Another survey conducted by Chongqing-based human resources website job.cq.qq.com found that over 70 percent of job seekers believe that having a second child would make females less popular in the job market, although two-thirds of the employers said the policy will make no difference in their employment of staff.

    Feng Lijuan, s senior expert on human resources at 51 job.com, a leading Chinese job finding platform, said she would not say “there is prejudice against career women.” Feng said Chinese women shoulder more family responsibility. “It is not only about maternity leave; a female employee might only fully get back to work after three to five years after having her first child.”

    Wang Yixin, a senior employment adviser, said the positive side is that more companies are trying to attract more talents by providing support to career women. “Different from before, it is not only employers choosing employees. Many talents, including professional career women, also choose employers.” said Wang. “According to our survey, many large companies are very open to their employees' choice of having a second child.”