用代入法解方程组$\left\{\begin{array}{c}2x+3y=7\\ 5x-y=9\end{array}\right.$先消去未知数（　　）最简便．

读一读，补全对话。

A. What about you?            
B. How old is your cousin?

C. Do you have any cousins?    
D. What is his name?
 Ann: Hi! Mike.____
Mike: Yes, I have one cousin.
Ann: ____
Mike: He's 9 years old. ____
Ann: I have one cousin, too. He's a boy.
Mike: ____
Ann: His name is Dan.He is 10 years old.

一只猴子去果园采桃子，第一天采了$\frac{1}{10}$，以后八天分别采了当天现有桃子的$\frac{1}{9}$，$\frac{1}{8}$，$\frac{1}{7}$…$\frac{1}{3}$，$\frac{1}{2}$，采了9天，树上只剩下10只桃子．树上原有桃子多少只？

“水国寒消春日长,燕莺催促花枝忙。风吹金榜落凡世,三十三人名字香。”这首《及第谣》中体现的制度正式诞生是在（     ）

根据图所示实验，不能得到的结论是（）