22. You can avoid tooth decay by brushing your teeth regularly.
(PREVENT)

21. Most stores will accept a credit card instead of cash.
(ALTERNATIVE)
 

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.