Fact 1: Some arthropods have special excretory structures. Fact 2: Terrestrial arthropods have an excretory system that conserves water and eliminates waste. Given the two facts above, what is the name of something some animals use to conserve water and elminate waste?

Choices: 1. it keeps an organism warm; 2. food and shelter; 3. Exfoliation; 4. an area swollen with pus; 5. swamp vegetation; 6. excretory structures; 7. reusing materials; 8. colder and wetter;
The answer is:
6.