Identify two structural features of purines and pyrimidines
purines
a. contain only three ring nitrogen atoms.
b. contain one heterocyclic ring.
c. contain four ring nitrogen atoms.
d. contain only two ring nitrogen atoms.
e. contain two heterocyclic rings.
pyrimidines
a. contain two heterocyclic rings.
b. contain only three ring nitrogen atoms.
c. contain only two ring nitrogen atoms.
d. contain four ring nitrogen atoms
e. contain one heterocyclic ring.

The answers are:

Purines:

C. contain four ring nitrogen atoms.

E. contain two heterocyclic rings.

Pyrimidines:

C. contain only two ring nitrogen atoms.

E. contain one heterocyclic ring.

Explanation:

Purines and Pyrimidines are nitrogenous bases which are the building blocks of nucleic acids (DNA and RNA).

Purines are composed by two fused heterocyclic rings, one of them is a 6-ring and the other is a 5-ring. Each ring contains two nitrogen atoms which form part of the ring. Thus, the nitrogen positions in purines are: 1', 3', 7' and 9'. Depending on the functional groups bonded to the two-ring structure, a purine base can be Guanidine (G) or Adenine (A).

The structure of Pyrimidines is a single heterocycle ring wich contains two nitrogen atoms in positions 1' and 3'. Depending of the functional groups, they can be: Cytosine (C), Thymidine (T) and Uracil (U, which is found in RNA).

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.