Answer:
The pressure differences on either side of the valve.
Explanation:
The heart valves open and close passively because of pressure differences on either side of the valve. When pressure is greater behind the valve, the leaflets are blown open and the blood flows through the valve. However, when pressure is greater in front of the valve, the leaflets snap shut and blood flow is stopped.
 Hope this helps!
have a blessed day 
 
        
             
        
        
        
D. Gravity and Distance. The distance earth is from the sun is what's keeping us from barreling into the mass of flame and heat. The distance between us and the moon is what keeps the moon from crashing into us. It could not be C, as the definition of inertia is: "a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force." Hope this helps :) 
 
        
                    
             
        
        
        
The answer in the space provided is impetigo. Impetigo usually comes after allergies and colds and happens usually in children. It could cause them to have raised lesions which are pink and water-filled. It usually appears after the person has red sores and the one that causes this is from staphylococcus infection.