Which of the following sentences is an academic definition?

A、For example, you are waiting in line to check out atthe grocery store and the lady behind you asks you to hand her a pack of gum.
B、Deja vu is a French word meaning "already seen", it has also been described as a feeling or experience that one has seen or done something before.
C、Suddenly you get an overwhelming feeling that you have been there in that exact same spot, talking to the same lady, even the same brand of gum.
D、Even though everything seems so familiar you know there is no way that could have happened before.