The answer is a. Nitrogen
        
             
        
        
        
In facilitated diffusion, substances move into or out of cells down their concentration gradient through protein channels in the cell membrane. Simple diffusion and facilitated diffusion are similar in that both involve movement down the concentration gradient.
        
                    
             
        
        
        
<span>Muscle color is related to muscle specialization because it indicates whether muscles are used more for aerobic or anaerobic metabolism. Aerobically respiring muscles that do work over a long period of time have more red oxidative fibers, while anaerobically respiring muscles that do work in bursts have more white glycolitic fibers.</span>
        
                    
             
        
        
        
Water is the most abundant molecule in the Earth’s atmosphere is false.