Answer:
Due to presence at early days of the planet.
Explanation:
We only find the remains of single celled organisms in the oldest fossils because the single celled organisms are the organisms that were present in the early days of this planet. These single celled microbes were present about 3.7 billion years ago while on the other hand, the oldest rocks are only a little older i. e. 3.8 billion years so these single celled organisms are considered as oldest fossils on this planet.
 
        
             
        
        
        
Dominance involves one allele masking another at the same locus whereas epistasis involves one locus masking a different locus.
Explanation:
Epistasis is a type of gene interaction in which one gene masks or modifies the expression of another gene at a distinct locus. 
Dominance refers to the gene interaction in which one allele of a given gene masks the expression of another alllele of same gene.
The comparison of dominance and epistasis give the following  information:
- Dominance  involves  allelic suppression while epistasis involves non-allelic suppression.
- Dominance involves a single pair of alleles while epistasis involves two pairs of allele.
- In dominance, the expression of recessive allele is suppressed while in epistasis, either recessive or dominant  non allele is suppressed.
- The effect of dominance is due to the dominant allele while epistasis may be due to dominant or recessive allele.
 
        
             
        
        
        
The two types of crust are oceanic and continental, the lithosphere is composed of earth's crust and upper most mantle
        
                    
             
        
        
        
I believe A is the answer, hope this helps. :)
please give brainliest.