Here are the correct answers circled for each sentence based on their structure:

1. **C. Complex**
- "We should walk to school so that we can reduce air pollution."

2. **C. Complex**
- "We shouldn't throw trash into rivers so that we can reduce water pollution."

3. **B. Compound**
- "We should grow more plants and we shouldn’t waste paper."

4. **A. Simple**
- "We were extremely sad when the party was over."

5. **B. Compound**
- "We should have cheap public transportation and we shouldn’t use plastic bags."

6. **C. Complex**
- "We should recycle trash so we can reduce land pollution."

7. **B. Compound**
- "We should create more green spaces like parks and playgrounds."

8. **A. Simple**
- "We should provide more trash cans in the park."

If you need further explanations or clarifications, let me know!