Answer:
A DOI or URL should be listed for an electronic source.
Step-by-step explanation:
Making references for electronic sources and print sources are different because while one has to do with information gotten from physical books, the other has to do with information gotten digitally.
In the case of a reference for a print source, it is important to cite the name(s) of the author, title of the book, etc while in the case of an electronic source, the URL or DOI is used to show ownership of the material cited.