n个连续自然数按规律排成下表，根据规律，2011到2013，箭头的方向依次为（   ）

（填空题：将答案填写在题中横线上。）
1926年，北伐战争中任北伐军总司令的是____ 。北伐战争的主战场是____ 。

    Ann is my classmate. She comes from New York, and she lives in Shanghai with her parents now. Every morning, she goes to KFC to have breakfast. Sometimes she buys a hamburger for me. She can't speak Chinese well. In the evening, her father often helps her with it. He's a teacher in our school. He can speak Chinese very well.

    Ann thinks the school life(生活)in China is very interesting. Ann likes reading Chinese books because she wants to know more about China. She tries(努力)to talk with her Chinese friends in Chinese. She hopes to speak Chinese well. She is very happy to live in China. She loves people here.

如图所示，是某质点运动的v﹣t图像，请回答：

已知函数f（x）= $\frac{ax}{{\left(x+r\right)}^2}$（a＞0，r＞0）

（1）求f（x）的定义域，并讨论f（x）的单调性；

（2）若$\frac{a}{r}$=400，求f（x）在（0，+∞）内的极值．