题干

若组成DNA分子的N元素全部为14N,则在试管中离心后的结果如图乙;若组成DNA分子的N元素全部为15N,则在试管中离心后的结果如图甲.现在将14N的DNA分子在15N标记核苷酸的培养基中连续复制2次,复制产物经离心后的结果图应该是(    )

A:

B:

C:

D:

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C

同类题2

阅读理解

    The Lifecycle of a T-shirt

    We all probably have a lot of T-shirts, but do you ever stop and think about the influence of a T-shirt on the planet? You'd probably be surprised to learn what's involved in the lifecycle of just one T-shirt.

    There are 5 major stages: material, production, shipping, use and disposal. The material stage involves farming, irrigating, fertilizing, harvesting and ginning. While cotton is a natural fiber and not as harmful to the environment as manmade fibers, it still takes a toll in the material and production stages. Commercial cotton farming uses a large amount of water, and the use of pesticides (杀虫剂) is widespread across the globe, especially in cotton farming. Studies have shown that farmers spend around $4.1 billion on pesticides annually, of which 25% was spent on cotton crops in the US.

    Once the cotton is grown and harvested, so begins the production stage: spinning, knitting, bleaching, dyeing, cutting, sewing, etc.——these processes also use a great deal of water and energy. Commercial dyes and bleaches are harmful pollutants and can eventually pollute groundwater.

    After the T-shirt is produced, it enters the transportation stage. This often involves overseas shipping. Take a look in your closet. Chances are that most of your cotton garments (衣服) are made in China or India. Garments can be shipped via plane, ship or truck…, all of which spill CO2 into the atmosphere. Calculations show that CO2 emissions from light trucks alone amount to 1.15 pounds per mile.

    Once the T-shirt reaches the retail market, it is purchased. This stage may seem like the least environmentally damaging part. But consider the number of times you've washed and dried your favorite T-shirt. Washing machines are certainly becoming more efficient. However, the average American household does 400 loads of laundry per year, using about 40 gallons of water per load. Such excessive water use is combined with the large amount of energy used by dryers.

    The final stage of life is disposal. This releases harmful emissions, or involves a landfill where cotton takes years to break down. Current US records show that an estimated 15% of clothes and shoes are recycled, which means that consumers send a shocking 85% of these materials to landfills.

    We all need new clothes every once in a while, but let's all try to keep in mind what goes into the production of clothing... It has a real impact on the planet.

    There are a lot of things you can do to help reduce your impact. Reuse and recycle clothes. If they're too worn out to wear, cut them up and use them as cleaning rags. Donate them to charity or another organization that recycles textiles. When possible, make an effort to buy organic cotton. Turn down the thermostat on your washer, and line dry your clothes when the weather will allow it.

同类题5

阅读下面的文言短文,完成下列小题。

      【甲】世有伯乐,然后有千里马。千里马常有,而伯乐不常有。故虽有名马,只辱于奴隶人之手,骈死于槽枥之间,不以千里称也。

       马之千里者,一食或尽粟一石。食马者不知其能千里而食也。是马也,虽有千里之能,食不饱,力不足,才美不外见,且欲与常马等不可得,安求其能千里也?策之不以其道,食之不能尽其材,鸣之而不能通其意,执策而临之,曰:“天下无马!”呜呼!其真无马邪?其真不知马也。——韩愈《马说》

      【乙】七年,入见,帝①从容问曰:“卿得良马否?”飞曰:“臣有二马,日啖刍豆数斗,饮泉一斛,然非精洁则不受。介②而驰,初不甚疾,比行百里始奋迅,自午至酉,犹可二百里。褫③鞍甲而不息不汗若无事然。此其受大而不苟取;力裕而不求逞,致远之材也。不幸相继以死。今所乘者,日不过数升,而秣④不择粟,饮不择泉,揽辔未安,踊跃疾驱,甫百里,力竭汗喘,殆欲毙然。此其寡取易盈,好逞易穷,驽钝之材也。”--节选自《宋史·岳飞传》

      【注释】①帝从容问曰:指宋高宗赵构,此次问话后,岳飞就被封为太尉。②介而驰 :介,装上鞍甲。   ③褫鞍甲:褫,脱去,卸下。④而秣不择粟:秣,喂食。