Answer:
Option C. is correct.
Explanation:
According to Chargaff's rules, DNA (Deoxyribonucleic Acid) from any species of an organism should have purine and pyrimidine bases in the ratio 
This ratio is very important for solving the structure of DNA.
As per this rule, DNA is double-stranded, and covalent bonds between identical nucleotides hold the strands together.
So, Option C. is correct.
Answer: the failure of one pairs of homologous chromosomes
Explanation:
Two ways:
1. In prophase I of meiosis the homologous chromosomes synapse and crossing over occurs. This switches sections of two of the sister chromatids so the outer two chromatids have the same mix of alleles as the parents and the inner two chromatids have new combinations of alleles.
2. Meiosis puts only one of each kind of chromosome in the gametes, selecting one of each homologous pair at random. Then when each gamete joins with the gamete from the other parent, there are practically infinite combinations of the alleles possible.
The answer is Starch. Hope this helped!
