Which of the following is an example of correct colon usage?

A. She brought three things: a book, a pen, and a notebook.
B. She brought three things a book, a pen, and a notebook.
C. She brought: three things a book, a pen, and a notebook.
D. She brought three things; a book, a pen, and a notebook.

Answer: She brought three things: a book, a pen, and a notebook.