Answer:
A)Fixation. Nitrogen in its gaseous form (N2) can't be used by most living things
B) it can not be used because it most be converted or, ’fixed’  to a more useable form. To do this it goes through a process called fixation.fixation is the change in a gene pool from a situation where there exists at least two variants of a particular gene (allele) in a given population to a situation where only one of the alleles remains. 
C) This form can be made more usable if it’s converted. It has to go through the process of fixation so the gene pool changes. 
Explanation:
 
        
                    
             
        
        
        
Answer; 
 The L-D reactions require light energy and water, and the L-IND reactions require ATP, NADPH and CO2. 
Explanation; 
-Photosynthesis is the process by which green plants use light energy from the sun, water and carbon dioxide to make organic molecules, such as simple sugars. 
-The process takes place in two phases; light dependent stage and light independent stage. 
-During light dependent stage light energy is used to break down water molecules into hydrogen ions and oxygen atoms. 
-Light independent reactions uses CO2 and the ATP and NADPH from light dependent reactions to produce sugar/ glucose. ATP is used to provide the energy for sugar synthesis and NADPH is used for electrons in the reduction of Carbon Dioxide into sugar.
 
        
                    
             
        
        
        
The answer is
D. Repeated trials
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This answer to this question is c