What aspect of the DNA structure allows to have so much genetic variability?

A)All organisms follow Chargaff's rules of A =T and G =C.
B)The bases connected to the sugar-phosphate backbone can be in any order.
C)The nucleotides are joined and produces a sugar-phosphate backbone.
D)There are 4 different nucleotide building blocks.
E)There is complementary base pairing between the two strands.