Final answer:
Adenine, thymine, cytosine, and guanine are the four nitrogenous bases that make up the "rungs" of a DNA molecule.
Step-by-step explanation:
The four nitrogenous bases that make up the "rungs" of a DNA molecule, connecting the two strands, are adenine (A), thymine (T), cytosine (C), and guanine (G).