根据汉语意思提示完成句子。

1. By 2008, the environment here will be ____________ (更加舒适).

2. Do you know what the ____________ (目的) of learning English is?

3. In our country, we will never _____________ (挨饿)

4. In the past, each family had _____________ (至少) three or four children in our country.

5. I think ____________ (空气污染) is the most serious problem in the world.

油机的基本结构主要由曲轴连杆机构、配气机构、供油系统、和冷却系统等部分组成。