There is a new park near Andy's home. It's fine today. Andy and his family are in the park now. On their left, there is a café. On their right, there is a big lake. There are many flowers and trees around the lake. There's a small hill behind the lake. Near the lake, there are two signs. One says, “Don't swim in the lake!” The other says, “No fishing!” But you can go boating on the lake. There is a beautiful garden in the middle of the park. There are green grass and beautiful flowers in it. There are some small shops between the lake and the garden. The park is so beautiful. Andy and his family like it very much.

元代设立陕西行省，将一直属于巴蜀故地的汉中地区划入陕西行省，使汉中进入以西安为中心的文化辐射圈。经过几百年的发展，今汉中地区虽然仍保留一些蜀地旧俗，但其西北关中氛围也更加明显了。对以上材料解释最合理的是（    ）

如图，AB、CD是⊙O的直径，AB∥DE，AC=3，则AE=____ ．

卢瑟福原子核式结构理论的主要内容有（　　）

双受精作用指的是（　　）