3. The purpose of writing this passage is to _______.

A. introduce the role of bacteria in children’s weight

B. analyze the influence of obesity on kids

C. give advice on how to lose weight quickly

D. explain the function of bacteria in foods

(二)

One event that helps educate young people about things they can do for others is National Youth Service Day (NYSD). Every year on this day, organizations all over the country celebrate young volunteers, educate young people about how they can help their country, and invite them to participate in service projects. Former Secretary of State Colin Powell said, “National Youth Service Day honors young volunteers and is an invitation to others to join hands and do our part as Americans.”

Founded in 1988, NYSD originally took place on a Tuesday. Now the event lasts from Friday until Sunday so that more people can participate in it. More than 200 organizations work together to create thousands of projects each year. Millions of young people across the United States take part in these service projects.

Long before the event, people, community groups, schools, and nonprofit organizations think of ways to meet their communities’ needs. Then, they plan projects for young volunteers. Projects include bringing food to hungry people, tutoring younger children and helping senior citizens. During the event, young people from the community do things like planting neighborhood gardens and cleaning up local parks.

The organizations that contribute to NYSD believe that volunteering and community service are important activities for people of all ages, but especially for young people. Young people who volunteer are more likely to do well in school, vote, and contribute to charities. More important, they help other people and make the world a better place for everyone.

31.(14分)以前,某乡镇地区的居民生活污水,以及食品厂、造纸厂等企业的工业有机废水都向一条河流中排放,使原本清澈的河水变得又黑又臭。在环保部门配合下他们对污水进行了如下处理后,情况大有改观。

   水从高处向下流形成的景观叫“跌水”。跌水充氧技术的原理是:生活污水首先进入沼气池,经过沼气池处理后的污水再用泵提升进入多级跌水池。跌水池分多格串行,内装组合填料,其借助生长在填料上的微生物去除有机物;经过跌水池的污水一部分回流入沼气池,另一部分进入水耕型蔬菜池。这项技术被称为是生物处理与生态处理相结合的、低能耗的污水处理新工艺。

污水   →  沼气池   →  跌水池   →  水耕蔬菜池   → 处理水

                            → 农田回灌

图  跌水充氧生物接触氧化技术工艺流程图

(1)以前,大量污水的排放使河水变得黑臭,从生态系统的稳定性的角度分析其原因是_______________________________________。

(2)沼气池中沼气发酵是一个极其复杂的生物化学过程,包括各种不同类型微生物所完成的各种代谢途径。如S-菌系分解乙醇产H2,H2对它继续分解乙醇有阻抑作用,而MOH-菌系可利用H2和CO2合成沼气中的甲烷,以上两种菌系的关系在生物学上称为__________________________。

(3)经过栽培水耕蔬菜的水中__________  元素(至少两种)明显减少。 

(4)水耕型蔬菜池还生活着一些细菌、原生动物和其他生物类群,实质就是一个小型的__________________________。水耕蔬菜如空心菜、水芹菜、茭白等的生长,对水中_____________(一类植物)的繁殖和生长起了竞争抑制作用,实现了水体净化,并进一步实现了生态系统的_____________多层次的循环利用。

(5)为了检验污水处理的效果,常测定BOD5(5日生化需氧量)的值,BOD5通常指20℃下,1L污水中所含有机碳源在微生物氧化时,5日内所消耗分子氧的毫克数。若假设原来100mL污水中的有机物含量为2.1g,等价于1.8g葡萄糖的耗氧量,污水处理后测得BOD5的值为190mg/L,则污水净化率达到_____________左右(以BOD5减少的百分率表示)。

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