“神农因天之时，分地之利，制耒耜，教民农耕。”材料中的“神农”是（　　）

   A naughty dog went into the back of a butcher’s shop(肉铺). He saw a big meaty bone. He decided to take it with him.  ____   He ran out of the shop, carrying the bone in his mouth, and rushed into the forest.____ He looked down from the bridge and he saw another dog! The dog was carrying a big meaty bone. He thought, “That bone is much better than mine. It's got more meat on it, and I think it's longer too!____  I'm going to take that bone!”
   So saying, he jumped off the bridge. He opened his mouth to bite(咬) the other dog, letting go of his bone. But as soon as he hit the water, the other dog disappeared. And his bone sank into the river.
    ____ .“You saw another dog and another bone, but they were just reflections! Now you've lost your bone, and got soaking (浸湿) wet too!”

在人类现代化的进程中，世界性大国发挥着重要作用，深刻影响着世界历史的进程。阅读下列材料，回答问题。

材料一：这位身材不高，有着钢铁般意志和敏锐头脑的思想巨人，正在创造着人类历史上的奇迹，他建立了第一个社会主义国家……

──《世界上下五千年》

材料二：1921年春，苏俄发生大面积灾荒，3000多万老百姓挣扎在饥饿的边缘……然而这个政策引起了农民的不满，一切富农甚至借机拿起武器，连十月革命中布尔什维克的坚强堡垒──喀踉施塔得的水兵们也支持农民的要求。

──《大国崛起》

材料三：1991年12月25日晚，苏联最后一任领导人戈尔巴乔夫发表电视讲话，宣布辞去苏联总统职务。同时，飘扬在克里姆林宫上空的苏联国旗徐徐降下。

（1）材料一中的他是谁？他领导的哪一次革命建立了“第一个社会主义国家”？

（2）为了挽救材料中描述的危机，苏俄又采取了什么政策？这一政策的内容是什么？

（3）这一震惊世界的历史事件说明了什么？推算一下苏联共存在了多久？