Here are the completed sentences with the appropriate words/phrases filled in:

1. **Pollution** is a serious problem everywhere.
2. We should avoid dumping waste into lakes and rivers to **reduce** water pollution.
3. We shouldn't throw away **single-use** products but recycle them.
4. **Carbon dioxide** is the amount of carbon dioxide we release into the environment.
5. We hope a lot more people join our **environmental program**.
6. People in my neighborhood are doing a lot to save **endangered species**.
7. This National Park provides a rich **habitat** for marine life.
8. Forests help release oxygen and absorb **carbon footprint**; they also provide homes for many species.
9. **Cutting down trees** is a serious environmental concern as it harms natural habitats.
10. An **ecosystem** may be a whole forest, or a small pond, and it can be of any size.