Answer :
If the nitrogenous base is made of two rings that means it's a purine(adenine, guanine), if it's made of one ring that means it's a pyrimidine(cytosine,thymine, uracil).
If the nitrogenous base is made of two rings that means it's a purine(adenine, guanine), if it's made of one ring that means it's a pyrimidine(cytosine,thymine, uracil).