Which one of the following significantly contributes to the specificity of base pairing in double-stranded DNA, i.e., limits base pairing to guanine-cytosine (G:C) or adenine-thymine (A:T)?

a. Base-stacking interactions.
b. Hydrogen bonding between bases.
c. The phosphodiester bonds between phosphate groups and deoxyribose sugar molecules.
d. The right-handedness of double-stranded DNA.
e. The predominant keto and enol conformations of A, T, G, and C.