题干

关于DNA分子的结构与复制的叙述中,正确的是(    )

①含有m个腺嘌呤的DNA分子复制n次需要游离的腺嘌呤脱氧核苷酸为(2n﹣1)×m个

②某DNA片段有300个碱基对,其中1条链上A+T比例为35%,则第3次复制该DNA片段时,需要780个胞嘧啶脱氧核苷酸

③细胞内全部DNA被32P标记后在不含32P的环境中进行连续有丝分裂,第2次分裂的每个子细胞染色体均有一半有标记

④DNA一条链被32P标记后,复制n次,子代DNA中有标记的占

A:①②

B:②③

C:③④

D:②④

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A

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    The kids in a village in Ethiopia wear dirty, ragged clothes. They sleep beside cows and sheep in huts made of sticks and mud. They have no school. Yet they all can chant the English alphabet, and some can make words.

    The key to their success: 20 tablet computers(平板电脑) dropped off in their Ethiopian village in February by a U.S. group called One Laptop Per Child.

The goal is to find out whether kids using today's new technology can teach themselves to read in places where there are no schools or teachers. The Massachusetts Institute of Technology researchers analyzing the project data say they're already amazed. “What I think has already happened is that the kids have already learned more than they would have in one year of kindergarten,” said Matt Keller, who runs the Ethiopia program.

    The fastest learner—and the first to turn on one of the tablets—is 8-year-old Kelbesa Negusse. The device's camera was disabled to save memory, yet within weeks Kelbesa had figured out its workings and made the camera work. He called himself a lion, a marker of accomplishment in Ethiopia.

With his tablet, Kelbasa rearranged the letters HSROE into one of the many English animal names he knows. Then he spelled words on his own. “Seven months ago he didn't know any English. That's unbelievable,” said Keller.

    The project aims to get kids to a stage called “deep reading,” where they can read to learn. It won't be in Amharic, Ethiopia's first language, but in English, which is widely seen as the ticket to higher paying jobs.