微语法填空温馨提示: 加黑部分为复现前面知识
1. Throughout the history, there are various theories(theory) about astronomy, which refer to solar system, atmosphere, gravity and so on. It is believed that our earth came into existence(exist) about 3 billion years ago. Everything on the earth has been changing, including the climate, however, its history is still a puzzle.
2. There are numerous man-made satellites(satellite) and asteroids floating(float) in the space. Some may crash onto the earth, and burn violently(violent) in the process. But most of them are harmless (harm), and some aerolites are of great value for biologists to study aliens. Also, spaceship made by us should avoid hitting them.
3. There exist so many chemical factories that our homeland are seriously polluted. Therefore, if we don’t take action in time, it will be difficult for us to save our land.
