题干

在艾弗里证明遗传物质是DNA的实验中,用 DNA酶处理从S型活菌中提取的DNA与R型菌混合培养,结果发现培养基上仅有R型菌生长。设置本实验步骤的目的是(       )

A:证明R型菌生长不需要DNA

B:补充R型菌生长所需要的营养物质

C:直接证明S型菌DNA不是促进R型菌转化的因素

D:与“以S型菌的DNA与R型菌混合培养”的实验形成对照

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D

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    It was once common to regard Britain as a society with class distinction. Each class had unique characteristics.

    In recent years, many writers have begun to speak of the 'decline of class ' and 'classless society ' in Britain. And in modern day consumer society everyone is considered to be middle class.

    But pronouncing the death of class is too early. A recent wide-ranging study of pubic opinion found 90 percent of people still placing themselves in a particular class; 73 percent agreeed that class was still a vital part of British society.; and 52 percent thought there were still sharp class differences. Thus, class may not be culturally and politically obvious, yet it remains an imprtant part of British society. Britain seems to have a love of stratification.

    One unchanging aspect of a British person's class position is accent. The words a person speaks tell her or his class. A study of British accents during the 1970s found that a voice sounding like a BBC newsreader was viewed as the most attractive voice. Most people said this accent sounds 'educated ' and 'soft '. The accents placed at the bottom in this study, on the other hand, were regional(地区的) city accents. These accents were seen as 'common ' and 'ugly '. However, a similar study of British accents in the US turned these results upside down and placed some regional accents as the most attractive and BBC English as the least. This suggests that British attitudes towards accent have deep roots and are based on class prejudice.

    In recent years, however, young upper midder-class people in London, have begun to adopt some regional accents, in order to hide their class origins. This is an indication of class becoming unnoticed. However, the 1995 pop song ' Common People ' puts forward the view that though a middle-class person may ' want to live like common people ' they can never appreciate the reality of a working class life.