综合
In passage
Three reasons for sea cow’s extinction
Firstly， over hunted by native Siberian people
Secondly, ecosystem disturbance make kelp(which sea cow was fed on) decreased
Thirdly, European fur trader hunt sea cows too.
In lecture
Firstly, sea cow is massive that a couple of sea cows may fulfill the need of people in a village for a month, thus they don’t need to hunt too much sea cows.
Secondly, decrease of kelp may influence other marine mammals such as wheals
Thirdly, the population of sea cow is small before the traders came, there is something else make the sea cow extinct.
以上语言用法什么的就是随意写的，估计很多错误...另外记忆内容可能不准，大概是这个意思
独立
agree or not
Educating children is a more difficult task than before because children spend too much time on cell phones, online games, and social networking Web sites.
