Answer:
One reason that Organism A and Organism C will be placed in different groups will be due  to their MODE OF NUTRITION.
As described in the question, Organism A has an autotrophic mode of nutrition which means that it has the capability to produce its own food. Organism C is a heterotroph which means it cannot produce its own food and hence will have different properties as compared to the organism A. 
 
        
             
        
        
        
A decrease in the overall volume of gases like hydrogen can prevent nuclear fusion in a nebula.   
Explanation:
Nuclear fusion: The reaction of combining two or more atomic nuclei to form one or more different atomic nuclei and subatomic particles is known as nuclear fusion. A single heavy nucleus is formed from two lighter nuclei. Helium is made by fusing Hydrogen atoms together.
Nebula:  A nebula is a giant cloud dust and gas in space. Nebulae are formed out of the gas and dust that are thrown out by the explosion of a dying star.
A decrease in the overall volume of gases like hydrogen can prevent nuclear fusion in a nebula.
 
        
             
        
        
        
I will answer the question accordingly so that it help you under stand it better.
Question A. 
During an extreme heat wave, plants with white flowers survive better.
What mechanism is in work here?
Answer: Mechanism of Natural Selection
 will work here. This is called survival of the fittest which means the best color which fits in harsh conditions will dominate over others and will survive for longer time.
What is the effect on the gene pool?
Answer: The Frequency of white allele will increases
 because white color is the best fit to extreme heat wave.
Question B.
A person uproots the five closest plants, which all happen to have white flowers.
What mechanism is in work here?
Answer: The mechanism of gene flow will work here as color of flower represents genotype of flower.
What is the effect on the gene pool?
Answer: The Frequency of white allele will increases because genotype of white flowers is increased.