Plagiarism—illegally using other people's ideas—can be avoided by
A. adding a bibliography.
B. proper documentation.
C. taking good notes.
D. using online sources only.
I'd say A. Adding A Bibliography because the definition of bibliography is this: A list of the books of a specific author or publisher, or on a specific subject.
"Collect this information for each Web Site:
author name.
title of the publication (and the title of the article if it's a magazine or encyclopedia)
date of publication.
the place of publication of a book.
the publishing company of a book.
the volume number of a magazine or printed encyclopedia.