下列能正确表示草原生态系统中草、鼠、蛇、鹰四种生物之间关系的食物链是（　　）

A．草→鼠→蛇→鹰

B．草→鹰→蛇→鼠

C．鹰→草→蛇→鼠

D．草←鼠←蛇←鹰

根据句意和汉语提示，填写恰当的短语完成下列句子。

1. _____ (首先), the computer room must be kept clean.

2. _____ (有时) he goes to the English club after school.

3. Too much work and too little rest can _____ (引发) illnesses.

4. I am happy that I bought the computer _____ (以最低的价格).

5. This book is specially _____(针对) teenagers.