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
 
        
                    
             
        
        
        
Velocity is explained as the *speed and *direction something might move.  Speed means moving and Direction means a certain way (not stationary.)
Hope this helps!
        
             
        
        
        
After finding the oxidation states of atoms, you identify the half reactions (option c).
The half reactions are given by the change of the oxidation states of the atoms.
For example if Cu is in the left side with oxidation state 0 and in the other side with oxidation state 2+, then there you have a half reaction (oxidation reaction). And if you have O with oxidation state 0 in the left side  and with oxidation state 2- in the right side, there you have other half reaction (reducing reaction).
        
                    
             
        
        
        
Answer:
His map of the ocean floor did not support his theory
Explanation:
- Alfred Wegner was a great scientist
- He observed the plate tectonics of earth and described it in a theory.
- It was dismissed after viewing his ocean floor