题干

社鼠出生一个月后即性成熟,科研人员对湖泊中一个岛屿的社鼠种群特征进行调查研究,部分结果如图,下列分析正确的是(  )

A:社鼠在夏秋季出生率大于死亡率

B:种群增大过程中,雄性死亡率高于雌性个体

C:迁入和迁出是影响社鼠种群密度的主要因素

D:下半年种群中幼年个体占比低于上半年

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D

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    Ben, at the age of 23 ,was the youngest man to ski solo to the North Pole. He dragged a 180-kilogramme sledge (雪橇) over 1,420 miles through the worst Arctic conditions. This year, Ben plans to ski solo from the Antarctic coast to the South Pole and back in the autumn, carrying all his supplies on his sledge.

    Ben Saunders was fired after persuading the firm to support his disorganized first adventure. “Everything that could go wrong did go wrong. We didn't get to the Pole, so we had no media interest. No one heard about it.”

    “People said it was impossible for me to get to the Pole. I said, “No, I can get there,” and I did. Self-belief, I see it as being a bit like a muscle — it's my belief that the more you stretch yourself the stronger it gets. If you never do anything that's uncomfortable or risky then your self-belief gets weaker. So that's one of the lessons I've figured out along the way.

    “My Antarctic adventure is just practicable and that's what is exciting to me. If I knew I could do it without too much bother, I wouldn't be interested." Why? “Personally I'm attracted by the human performance element to it. Not that long ago, running a marathon was seen as the top point of human attempt, and now I wouldn't be that surprised if my mum said she was going to run one."

    People's horizons are changing. “I'm not particularly gifted. I've just chosen this one goal to achieve and I've been working hard to realize it. And that's the thing that attracts me: with enough training and enough determination, enough focus and preparation, how far can we go? And I dont think I've found out yet.”