I dont fully understand the question, and it doesnt seem to be in the book

do you mean purines and pyrimadines?

I guess it means why does A bind to T, and C with G. It it because of how the hydrogen bonding sites of the two bases align. Aside from adenine, each base has at least one =O group. These have to line up with a hydrogen on the other base. For adenine a N atom within the ring binds to a hydrogen on thymine. When two other bases pair up (called 'non-canonical' base pairing) the hydrogen bonding sites are usually further apart, and this weaken the bond between the bases.

Thanks, I basically described the basic difference in the bonding sites.