The balanced reaction is
 Ca + 2H2O → Ca(OH)2 + H2 
Here this reaction can be compared with 
A + BC  ---> AB + C
So here one reactant (A) is accepting a group which is being given by another compound (BC) however the A is not giving any group / element or ion
So this single displacement
Similarly in the given reaction
the anion OH- is only being replaced 
The element Ca accepts OH- and H2O loses the same group to form new element H2
So the correct answer is 
Single replacement also known as single displacement
 
        
                    
             
        
        
        
Global warming, Cosmic Background radiation (even though most is blocked not ALL), and pollution. 
 
        
             
        
        
        
Answer:
B
Explanation:
<em>Both plain and plateau have flat surfaces. However, a plain is located in a low-lying area while a plateau is located on an elevated area. In essence, a plateau can be viewed as an elevated plain or a plain that is bordered by cliffs.</em>
The correct option is B. 
 
        
             
        
        
        
a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
  -  a person or thing that precipitates an event.