3. **If** we continue to throw waste into the ocean, the aquatic animals can die.

4. **Because** they plant more trees, they want our environment better and cleaner.

5. **There are a lot of factories here** because the air is polluted.

1. **Đáp án:** It's important to protect endangered animals to save our environment.  

* We continue to throw waste into the ocean, if the aquatic animals can die.
   * Corrected: We continue to throw waste into the ocean, so the aquatic animals can die.
   * Explanation: "So" is used here to show the result or consequence of the action of throwing waste into the ocean.
 * They plant more trees. They want our environment better and cleaner.
   * Corrected: They plant more trees because they want our environment better and cleaner.
   * Explanation: "Because" is used to introduce the reason why they plant more trees.
 * The air is polluted because there are a lot of factories here.
   * This sentence is already correct. "Because" is correctly used to show the cause of the polluted air.
Part 2
 * Unless we reduce energy consumption/ pollution levels will remain high.
   * Corrected: Unless we reduce energy consumption, pollution levels will remain high.
   * Explanation: "Unless" introduces a condition. It means that if we don't reduce energy consumption, then pollution levels will stay high.
 * Unless companies follow eco-friendly practices/ the Earth will be healthier.
 * Corrected: Unless companies follow eco-friendly practices, the Earth will not be healthier.
 * Explanation: Again, "unless" introduces a condition. If companies don't follow eco-friendly practices, the Earth's health will suffer.
Exercise 17
 * Protecting endangered animals is important to save our environment.
   * Corrected: It's important to protect endangered animals to save our environment.
   * Explanation: This sentence is already quite natural, but you could also say: "Protecting endangered animals is crucial for saving our environment."
 * We should recycle plastic bottles to protect the environment.
   * Corrected: Plastic bottles should be recycled to protect the environment.
   * Explanation: This sentence emphasizes the action of recycling plastic bottles.