Here are the correct answers for the exercise:

1. You shouldn't eat sweets, **or** you will have a toothache.
2. I forgot to **wear** suncream today and I got a sunburn.
3. The doctor advised him to exercise more, **but** he didn't follow his advice.
4. She eats so much fast food that she's **overweight**.
5. Using your mobile phone too much is not good **for** your eyes.
6. We can **avoid** some diseases by keeping ourselves clean.
7. Exercise is key **to** good health.
8. Vitamin D has been shown to fight diseases, improve bone health, and prevent **depression**.
9. Kids and teens who are bored, stressed, or depressed may eat **more** to cope with negative emotions.
10. I'm allergic to seafood, **so** I can't eat lobsters and shrimps.
11. If you want to **lose** weight, you should follow a low-fat diet.
12. She stays in **shape** by exercising daily and eating well.