| A.To express his doubts. |
| B.To compare different ideas. |
| C.To introduce points for discussion. |
| D.To describe the conditions on Earth. |
| A.Humans are nature-born to do so. |
| B.Humans have the tendency to fight. |
| C.Humans may find new sources of food. |
| D.Humans don’t like to stay in the same place. |
| A.Our genetic makeup. |
| B.Resources on the earth. |
| C.The adaptive ability of humans. |
| D.By-products in space exploration. |