先看一看a、b、c、d中哪些是直线，再用三角尺检验一下．

阅读短文并选择
    Miss Lee is 21 years old. She likes students and her work. From Monday to Friday she stays at school and gives the students lessons. She has much work to do, but she often plays games with her students after school. On Sundays, she drives to her parents' home and stays with them. Sometimes she goes to see her friends on Sundays. She drives back to school on Monday morning. She's a good girl.

长时间行走使脚掌磨出了水泡，几天后水泡消失。此时水泡中的液体主要渗入

动物促进生态系统的物质循环，正确的理解是（    ）

城镇自来水厂生产流程可用如图表示：自来水生产中通常用明矾做絮凝剂，其中明矾的作用是____．流程中除去水中固态杂质的设备是____；投药（多为ClO₂）的作用是____．