.--- Hurry up. The bus is coming. --- Oh, no. We mustn’t cross the street_______the traffic lights are green.
6.Newly-developed materials for spaceships can___________extreme weather conditions.
5.A good teacher is ________who is always doing things for the good of the students.
4.The first time I met David in the Central Park, both of us felt that we________each other for ages.
3.---How much do I owe you for this beautiful vase? ---____.It’s a gift.
2.Despite the fact that Betty is a lovely girl, she___________be extremely difficult to work with sometimes.