One solution to this problem now is sustainable development.
| A．discussed | B．discussing |
| C．to be discussed | D．being discussed |
6．Newly-developed materials for spaceships can___________extreme weather conditions.
| A．hold | B．support | C．stand | D．Carry |
5．A good teacher is ________who is always doing things for the good of the students.
| A．that | B．those | C．the one | D．One |
4．The first time I met David in the Central Park, both of us felt that we________each other for ages.
| A．knew | B．have known | C．had known | D．Know |
3．---How much do I owe you for this beautiful vase?
---____.It’s a gift.
| A．You’re welcome | B．That’s right | C．With pleasure | D．Forget it |
2．Despite the fact that Betty is a lovely girl, she___________be extremely difficult to work with sometimes.
| A．must | B．should | C．would | D．Can |