若a=$\frac{1}{3-\sqrt{8}}$﹣$\frac{1}{\sqrt{8}-\sqrt{7}}$+$\frac{1}{\sqrt{7}-\sqrt{6}}$﹣$\frac{1}{\sqrt{6}-\sqrt{5}}$，则a的值所在范围为（　　）

根据达尔文的进化学说，人类起源于（    ）

 $1-\frac{3}{7}+\frac{4}{7}=1-1=0$ 。

学校生物组收集蝴蝶标本68只，蜻蜓标本43只？蜻蜓标本比蝴蝶标本少多少只？

听短文，在短文中填写所缺单词
    Tom is a____boy. He is eleven years old. He's two years younger than his____, Jim. Tom is 150cm and Jim is 160cm. Jim is 50 kilos and Tom is 60 kilos. Tom wants to lose some weight, so he tries not to eat too much____. He also____more exercise than____.
    ____morning he gets up earlier than Jim and he goes____for____minutes. He usually does it after school. After dinner, Tom often goes for a ____, and Jim likes____at home. Now Tom is much____than Jim. Jim thinks he____more exercise now. He and Tom will do exercise tighter.