Only after he had spent several nights working on the math problemto wonder whether the problem was wrongly set.[ ]

We________on this project for four hours.Let’s have a rest

here is less and less coal and oil,scientists are exploring new ways of making use of ______ energy, such assunlight, wind and water for power and fuel.

Mary wrote an article on ____the team had failed to win the game .

It is easy for the students toschool because of the new subway in the city.