Here are the appropriate articles filled in:

1. Cindy recommended **a** good dentist, but **the** dentist doesn't have any openings for two months.
2. Sam recommended **a** book to Lisa. She didn't like **the** book at all.
3. There's **a** big bear in the backyard. I think **the** bear is looking for something to eat.
4. Neil Armstrong was the first person to walk on **the** moon.
5. You could feel **the** earth shake as the truck came closer.
6. The sun rises highest in **the** sky during the summer.
7. The helicopter hovered a couple of hundred metres above **the** ground.
8. Their products are sold throughout **the** world.
9. The government should do more to protect **the** environment.
10. Kevin talked about his hopes for **the** future.
11. As far as we know, **the** universe was created in the big bang.
12. I enjoyed myself in the quiet pace of life in **the** countryside.
13. Honey, where's **the** vacuum? I can't find it. It was in **the** closet, but now it's not there.
14. A: Why was today's class cancelled? - B: Because **the** teacher is sick.
15. A: I want to change the channel. - B: OK, **the** remote control is over there.
16. A: Oh no, where is it? - B: Don't worry, **the** key is in my pocket.