Answer:
water can maintain the temperature on the earth surface
 
        
             
        
        
        
My guess would have to be hydropower, since water isn't an un natural substance 
 
        
                    
             
        
        
        
Answer:  This is a positive feedback loop.
Explanation:
The protein digestion in the body is an example of the positive feedback. The gastric secretions is stimulated chiefly by the action of three chemicals.
These chemicals are acetylcholine, histamin and gastrin. These chemicals stimulate the parietal cells to secrete HCL and intrinsic factors. 
The chief cells secrete pepsinogen in response to the gastrin and ACh. As the dietary proteins are digested, it breaks down the amino acids and peptides.
This stimulates the G cells of the stomach to secrete more gastrin. This is how the positive feedback loop for the protein digestion works.
 
        
             
        
        
        
Answer:
Heat from the sun travels to earth by Electromagnetic Radiation.