The trough and the hill part
        
             
        
        
        
  Mass  of  electrons  are not  included when  calculating  the  atomic  mass  of   element.  Atomic  mass  of  the   element  is  equal  to   proton  +  neurons.  Example  the    the  mass  number  of  Nitrogen can   be  calculated as      7 protons  +  7  neutrons  to  give 14 .  thus  nitrogen  has  a  mass  number  of  14
        
             
        
        
        
I think the answer the students average is 15 km?
 
        
             
        
        
        
Answer:
the answer is D. stress 
If a stress is applied, the equilibrium shifts to relieve this change in conditions.