Here are the best options for each blank based on standard English usage:

6. You should put **A. lip balm** on your lips to protect them from sun, wind, etc.

7. We must find a way to **C. keep** similar problems in future.

8. Don't drink too much milk, **A. or** you will gain weight.

9. The doctor advised him to play more sports and **A. more** computer games.

10. Who's taking care **C. of** the children while you're away?

11. To **B. stay** healthy, you need between 1,600 and 2,500 calories.

12. You shouldn't **B. pop** spots when your hands are dirty.

13. We shouldn't spend much time **B. to watch/play** TV and games.

14. My brother eats a lot of junk food, so he **D. puts** on a lot of weight.

15. **C. Diet** is one of the main reasons why the Japanese have long lives.