20. They failed to find out a solution to the problem.
→They didn’t succeed 

19. He was sorry he hadn’t said goodbye to her at the airport.
→He regretted 

18. It takes six hours to drive from here to London.

It is...

17. We’ve run out of petrol.

16. That was a silly thing to say!

15. A man I don’t know told me about that.

14. People believe that the Chinese invented paper in 105 A.D.

13. These young men became doctors after six years training.

12. Because of the absence of the atmosphere on the moon, the sky
there is black, just as in space.

11. As people use a lot of wood-pulp, many trees are cut down.