题干

生物的某些变异可通过细胞分裂某一时期染色体的行为来识别。甲、乙两模式图分别表示细胞减数分裂过程中出现的“环形圈”、“十字形结构”现象,图中字母表示染色体上的基因。下列有关叙述正确的是                                            (   )

A:甲、丙两种变异类型的分别属于染色体结构变异、基因重组

B:甲图是由于个别碱基对的增添或缺失,导致染色体上基因数目改变的结果

C:乙图是由于四分体时期同源染色体非姐妹染色单体之间发生交叉互换的结果

D:甲、乙两图常出现在减数第一次分裂的前期,染色体数与DNA数之比为1:2

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D

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    Some years ago, writing in my diary used to be a usual activity. I would return from school and spend the expected half hour recording the day's event, feelings, and impressions in my little blue diary. I did not really need to express my emotions by way of words, but I gained a certain satisfaction from seeing my experiences forever recorded on paper. After all, isn't accumulating memories a way of preserving the past?

    When I was thirteen years old, I went on a long journey on foot in a great valley, well-equipped with pens, a diary, and a camera. During the trip, I was busy recording every incident, name and place I came across. I felt proud to be spending my time productively, dutifully preserving for future generations a detailed description of my travels. On my last night there, I wandered out of my tent, diary in hand. The sky was clear and lit by the glare of the moon, and the walls of the valley looked threatening behind their screen of shadows. I automatically took out my pen….

    At that point, I understood that nothing I wrote could ever match or replace the few seconds I allowed myself to experience the dramatic beauty of the valley. All I remembered of the previous few days were the dull characterizations I had set down in my diary.

    Now, I only write in my diary when I need to write down a special thought or feeling. I still love to record ideas and quotations that strike me in books, or observations that are particularly meaningful. I take pictures, but not very often—only of objects I find really beautiful. I'm no longer blindly satisfied with having something to remember when I grow old. I realize that life will simply pass me by if I stay behind the camera, busy preserving the present so as to live it in the future.

    I don't want to wake up one day and have nothing but a pile of pictures and notes. Maybe I won't have as many exact representations of people and places; maybe I'll forget certain facts, but at least the experiences will always remain inside me. I don't live to make memories—I just live, and the memories form. themselves.