Prior to the solving of the structure of DNA, Erwin Chargaff calculated the amounts of the four nucleotides (A, T,G,C) and made which of the following conclusions?

A. All four nucleotides are found in equal amounts within a single cell type.

B. Same levels of each of the four nucleotides are found within differing cell types, tissues and organisms. 

C. The level of the four nucleotides varies between cell types but the amount of purine and pyrimidine  nucleotides is equal within a given cell type. 

D. Within a given cell all four nucleotides are present in disimilar amounts.