Final answer:
Adenine and guanine are classified as purines, while cytosine and thymine have a single heterocyclic ring structure and are known as pyrimidines.
Step-by-step explanation:
The nitrogenous bases adenine (A) and guanine (G) are classified as purines.
The pyrimidines, cytosine (C), and thymine (T) have a single heterocyclic ring structure.
Therefore, the pyrimidines have a single carbon-nitrogen ring structure and are known as pyrimidines.