Erwin Chargaff conducted a series of experiments in which he was calculating the amounts of dissimilar nucleotides that were present in dissimilar tissues and organisms. These studies were critical to Watson and Crick's efforts to verify the structure of DNA. Which of the following best explains the relationships that Chargaff identified?

A. The amount of every nucleotide is constant across tissue types and organisms.

B. All four nucleotides are found in equivalent amounts within a single cell type.

C. The amount of an individual nucleotide within a cell is totally independent of the other three nucleotides.

D. Within an individual cell the total amount of purines and pyrimidines is equivalent.