Identify two structural features of purines and pyrimidines.

purines

__contain two heterocyclic rings.
__contain only two ring nitrogen atoms.
__contain four ring nitrogen atoms.
__contain one heterocyclic ring.
__contain only three ring nitrogen atoms.

pyrimidines

__contain one heterocyclic ring.
__contain four ring nitrogen atoms.
__contain only three ring nitrogen atoms.
__contain two heterocyclic rings.
__contain only two ring nitrogen atoms.