Answer:
Complex  System
Explanation:
Given that, a descriptive scientific investigation is one of the three main types of investigation which formulates and quantify the natural phenomenon.  This natural phenomenon oftentimes involves Complex System, such as microscopic organisms, thereby, scientists often make observations to understand the interacting parts of this COMPLEX SYSTEM
Hence, the right answer is a COMPLEX SYSTEM
 
        
             
        
        
        
Explanation:
A point of temperature at which both solid and liquid state of a substance remains in equilibrium without any change in temperature then this temperature is known as melting point.
For example, melting point of water is 
. So, at this temperature solid state of water and liquid state are present in equilibrium with each other. 
Therefore, when a 100 g of given pure metal in solid state is heated at its exact melting point which is 
 then some of the solid will change into liquid state but the temperature will remains the same.   
 
        
             
        
        
        
The answer is C, i just took the k-12 test~
        
             
        
        
        
Potential energy and height; best guess;)
        
                    
             
        
        
        
What do you mean by that Oh wait do you mean like 2×2=4 or 2+2=4 something like that maby?? I am still confused?? XD