题干

人体细胞内有46条染色体,次级卵母细胞中,姐妹染色单体、染色体、四分体、DNA依次有 


 

A:46、23、0、46

B:23、46、0、23

C:46 、23、0、92

D:184、92、46、46

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A

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    Last year I ruined my summer vacation by bringing along a modern convenience that was too convenient for my own good: the iPad. Instead of looking at nature, I checked my email. Instead of paddling a small boat, I followed my Twitter feed (推特简讯). Instead of reading great novels, I stuck to reading four newspapers each morning. I was behaving as if I were still in the office. My body was on vacation but my head wasn't.

    So this year I made up my mind to try something different: withdrawal (退出) from the Internet. I knew it wouldn't be easy, since I'm bad at self-control. But I was determined. I started by giving the iPad to my wife.

    The cellphone signal at our house was worse than in the past, making my attempts at cheating an experience in frustration (沮丧). I was trapped, forced to go through with my plan. Largely cut off from e-mail, Twitter and my favorite newspaper websites, I had few ways to connect to the world except for radio and how much radio can one listen to, really? I had to do what I had planned to all along: read books.

    This experience has had a happy ending. With determination and the strong support of my wife, I won in my vacation struggle against the Internet, realizing finally that it was I, not the iPad, that was the problem.I knew I had won when we passed a Starbucks and my wife asked if I wanted to stop to use the Wi-Fi. “ I don't need it,” I said.  

    However, as we return to post -vacation life, a harder test begins: Can I continue when I'm back at work? There are times when the need to know what's being said right now is great. And I have no intention of giving up my convenience completely. But I hope to resist the temptation (诱惑) to check my e-mail every five minutes, which leads to checking my Twitter feed and a website or two.

    I think a vacation is supposed to help you reset your brain to become more productive. Here I hope this one worked.