13. Earth will be too polluted _________ (live) on.

12. By 2100, we will spend only a few hours _________ (travel) to Mars in space shuttles.

11. The news makes her __________ (feel) sad.

10. Before the Project of Sanxia, many _________ (移民) left their hometown.

C. 用所给动词的正确形式填空。

9. A space sleeping bag is used to be fixed to the walls to prevent __________(飘浮).

8. Power pack can __________(提供) energy for the spacesuit.

7. Humans need food, water and air to __________(生存)

6. ___________(地球引力) will be a problem for the early settlers on Mars.

5. ____________ (science) are working hard to make the dream become true.

　 B. 根据所给中文和句意，填入正确的单词。

4. There will be no ___________ (pollute) on Mars.