B. 30% because the paired nitrogenous bases are always the same percentage. The four nucleotides are either purines, which are double-ringed structures, or pyrimidines, which are single-ringed structures. These are referred to as bases. These create base pairs formed by complementary nucleotides (purine with pyrimidine) connected by hydrogen bonds. Adenine will only pair with thymine, and cytosine will only pair with guanine. The hydrogen bonds cause the twisting that is characteristic of a double helix. The paired bases are always the same percentage. For example, in a sample of double stranded DNA where the nitrogenous base cytosine is at 40%, then the paired base guanine will be at 40% too.
