公元前16世纪比公元前8世纪 (     )

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    I have a good friend. 1 Chinese name is Da Shan and his 2 name is Mark Rowswell. He 3 from Canada. He can 4 English and French (法语). And he speaks Chinese very 5. He likes China very much. Now he teaches English 6 Beijing. Da Shan 7 a big nose, big eyes 8 short brown hair. He has a happy family. His wife (妻子) is from Sichuan, China. They 9 two children. Da Shan's father and mother are in Canada. But 10 like China, too. And they often come to

China.

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．

 粗盐中含有少量不溶性杂质（泥沙），还含有少量可溶性杂质（Na₂SO₄、CaCl₂、MgCl₂）．现以粗盐为原料制取精盐，生产过程如图所示