Answer: 
1.) stromatolites
2.) photosynthesis
3.) Ordovician
4.) Cambrian
5.) oxygen
6.) As Earth cooled, carbon dioxide dissolved in the early ocean. Cyanobacteria consumed carbon dioxide and converted it to oxygen. Methane was removed from the atmosphere as oxygen was produced, eventually leading to the Cambrian Explosion.
7.) Metabolic processes that use oxygen produce more energy than anaerobic metabolism. Simple diffusion of oxygen would limit the size of a multicellular eukaryote, but animals evolved ways to transport oxygen to their tissues, which permitted an increase in size.
Explanation:
From Penn 
 
        
             
        
        
        
Because it requires air, or oxygen to complete. Aerobic means with air. Therefor, cellular respiration is an aerobic process. I hope this helps, if you have a question, feel free to ask ! 
        
             
        
        
        
The small intestines have a protective lining that is resistant to acids