题干

硒元素是人体必须的微量元素,具有抗癌作用,它的相对原子质量为79,其原子的核外电子数为34,则它的质子数为(   )

A:34

B:45

C:79

D:113

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答案(点此获取答案解析)

A

同类题1

阅读理解

    Sunscreens (防晒霜) can offer protection from two types of ultraviolet (UV) rays (紫外线): UVB, which according to the Skin Cancer Foundation, are the chief cause of sunburn and play a key role in the development of skin cancer, and UVA, which get through the skin more deeply and are largely responsible for skin aging and wrinkling, as well as skin cancer risk. The SPF (防晒指数) measurement on sunscreens refers to the UVB protection level — and the number may not mean what you think it does. SPF 15 doesn't protect you from 15% of UVB rays, just as SPF 100 doesn't protect you from 100% of the rays. In fact, as you get into the higher numbered SPF sunscreens (think: 50 and above), the difference in UVB protection is actually pretty small.

    Theoretically, though the increase in UVB protection with a super-high SPF sunscreen may be not worth considering, it may help, and certainly can't hurt, to use it — that is, as long as you do so correctly. “The only downside is whether you're giving yourself a sense of false security,” Dr. Gohara says.

    Indeed, as Dr. Wang also pointed out, when people choose increased SPFs, they may actually feel so protected by the product that they finally ignore proper sun protection. “When people use really high SPF products, they tend not to reuse, they tend to stay out in the sun much longer, and they don't use other protective measures such as clothing and hats,” he says. Regardless of any increase in protection that a high SPF may or may not offer, one thing is certain: It doesn't give your body a free pass from sunburns, aging, or cancers, and it doesn't protect you from those things for any greater length of time than the lower SPF options. Both Dr. Wang and Dr. Gohara emphasized that, no matter what the SPF is, it's important to reapply your sunscreen every two hours.

同类题2

 阅读下列材料,回答问题。
材料一 这时,一方面,社会经济的发展,各地联系的加强,民族联系的加强“四海之内若一家”,为统一提供必要的社会基础。另一方面。秦国变法比较彻底.政权巩固,经济发展,国富兵强,在实力对比上较之山东六国有着明显的优势,已经有了进行统一战争的可能性。充分运用这些有利条件.秦王政“奋六世之余烈,振长策而御宇内”。果断地发动大规模的兼并战争。从秦王政十七年(公元前230年)灭韩起,至二十六年(公元前221年)灭齐止,前后只用了十年时间,便“初并天下”,统一了六国。     
——白寿彝主编《中国通史》
材料二 汉武帝在位5 4年,是中国古代统治年代比较长的帝王。汉武帝时代,西汉王朝开始进入全盛时期。汉武帝表现出雄才大略的政治思想和政治实践,在历史上留下了深刻的印迹。在他统治期间,以汉族为主体的统一的多民族国家得到空前的巩固,汉文化的主流形态基本形成。  
——张岂之主编《中国历史·秦汉魏晋南北朝卷》
材料三 根据工业复兴法,成立了有大银行家、工业家和经济学家参加的全国工业复兴局,由国家监督生产,调节各企业主之间的关系……根据法案还确定由国家调节雇主同工人之间的关系,确定最高工时和最低工资应签订集体合同事宜……(罗斯福新政)为以后美国和其他许多西方国家的资产阶级政府所效法。
——黄安年《罗斯福新政的历史地位和阶级性质》