Well it transports white blood cells to infected areas.
 
        
             
        
        
        
Answer A: When their separation increases.
Hope this helps! :D
        
             
        
        
        
The mass of the truck is needed because momentum=mass×velocity.
        
             
        
        
        
Answer:
The answer is 
A. Switch 
Explanation:
A switch is a device for making and breaking current in an electric circuit, it is mostly made of a non conducting material to prevent the user from being electrocuted, switches comes in range of types depending on the area of applications.