按一对夫妇生两个孩子计算，人口学家统计和预测，墨西哥等发展中国家的人口翻一番需20～40年，美国需40～80年，瑞典人口将会相对稳定，德国人口将会减少。预测人口的这种增减动态主要决定于(    )

    What do you think the future will be like? Here are some answers from some students.

    Mary: Everyone will have a computer at home. There will also be many machines in the future. Then people will work only two or three days in a week.

    Jack: The weather in the future will be very hot. People can't live on the earth. They will have to look for (寻找) another planet to live on.

    Linda: I think the life in the future will be nice. There will be a lot of flowers and trees. And cities will look like parks.

    Henry: Every family will have a family doctor, so people will be seldom ill (很少生病) They will live to 120 years old.

如图所示，清洗楼房光滑玻璃的工人常用一根绳索将自己悬在空中，工人及其装备的总重量为G，悬绳对工人的拉力大小为F₁，墙壁对工人的弹力大小为F₂，若工人增加悬绳的长度缓慢下移，则（   ）

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         至于夏水襄陵，沿溯阻绝。或王命急宣，有时朝发白帝，暮到江陵，其间千二百里，虽乘奔御风，不以疾也。
        春冬之时，则素湍绿潭，回清倒影。绝巘多生怪柏，悬泉瀑布，飞漱其间，清荣峻茂，良多趣味。
        每至晴初霜旦，林寒涧肃，常有高猿长啸，属引凄异，空谷传响，哀转久绝。故渔者歌曰：“巴东三峡巫峡长，猿鸣三声泪沾裳。”