The one that will change the velocity of a periodic wave is : 
B. Changing the medium of the wave
Waves is always determined by the properties of the medium, which means that changing the medium will change the velocity of the wave
hope this helps
        
                    
             
        
        
        
Answer:The energy of the wave by a factor of 4
Explanation:
 
        
             
        
        
        
1).  The little projectile is affected by friction all the way through the block.
Friction robs some kinetic energy.
2).  The block is affected by friction as it scrapes along the top of the post.
Friction robs some kinetic energy.
3).  The block is also affected by friction with the air (air resistance) as it
falls to the ground.  Friction robs some kinetic energy.
        
             
        
        
        
Answer:
smaller one
Explanation:
even though he is moving quicker doesn't mean he will be packing more force in the collision 
 
        
             
        
        
        
Answer:
here as we increase the distance the intensity will decrease and hence the amplitude of the electric field will decrease and vice-versa
Explanation:
As wee know that the amplitude of the wave will decide the energy of the wave
Here we know that energy density of electromagnetic wave is given as

now we have

so here we can say that intensity of the wave at the given distance from the source is given by formula

so here as we increase the distance the intensity will decrease and hence the amplitude of the electric field will decrease and vice-versa.