That's a component of a phospholipid, which is a lipid.
        
             
        
        
        
Combines photosynthesis and heterotrophic nutrition
        
             
        
        
        
I think the correct answer would be strong gravity. The factor that allows the earth to hold onto water would be the strong gravitational pull of the earth. In fact, it is what holds everything onto earth not only water. Hope this answers the question.