Newtons 3.law: Action = Reaction
If a body exerts a force on a rope of 400 N the rope exerts a force on the body of 400N also. So the tension in the rope is 400N. See pictures below.
 
        
        
        
No force is necessary to keep a moving object moving (in a straight line at a constant speed).
        
             
        
        
        
It's total kinetic energy
        
             
        
        
        
Answer:
20m/s
Explanation:
it covers 20 metres in a second
 
        
             
        
        
        
Answer:
D.
a control group
Explanation:
In a scientific experiment such as the one above, there is an experimental group and a control group. The experimental group is the group that receives the treatment while the control group does not receive any treatment. The control group helps the researcher to observe if the treatment had any significant effect. 
In this case, it will help Alan and Monica to determine if fertilizer X actually had an effect on the plant. Therefore, the pot with o grams of fertilizer in it is the control group.