Creating a template: How can I make a style that auto inserts text

What I am creating is a quiz template, so the idea is that it would prompt
the user as to what piece was next. Right now, the quiz styles default all
the way through, so when they hit Enter, it defaults to OptionA, then when
they hit Enter again, it defaults to OptionB. When they type in the text of
the last option and hit Enter, it defaults to the explanation para, and when
it does that, I'd like it to automatically insert the text "Explanation:" to
prompt the user. I don't think Word can do that, but are there add-on tools
that will? Any help or suggestions are appreciated!