阅读下面短文，从短文后各题所给四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。

    I went to Australia for summer holiday with my parents last year. I was so excited about the 1.

    We did many things there. The first day, we 2 Sydney. We had some 3food for lunch there. After lunch，we went to a big 4 . We saw tigers and koalas. The animals weren't 5 of people.

    6that, we had a walk and we 7 at a restaurant. We went back to the 8 to rest at 9: 00 in the evening. I was very 9. But I really 10 myself. I hope to go there again.

森林遭到持续干旱时，树木往往会扩展根系的分布空间，以保证获得足够水分，维持生态系统的正常功能．这反映了森林生态系统（  ）

假定当A商品的互补品价格上升10%时，A商品需求变动量为20单位；当A商品的替代品价格下降10%时，A商品需求变动量为30单位。如果其他条件不变，当A商品的互补品价格上升10%、替代品价格下降10%同时出现时，那么，A商品的需求数量                                                               

下列原子或离子核外电子排布不属于基态排布的是(　　)

定义新运算：x*y=$\frac{x+2y}{x-y}$，求a*b×b*（﹣a）．