Is it that We "Know" that the New Testament is Mt, Mk, Lk, Jn, letters of Paul etc. or We've Been Told it is so?

[Secret Alias]

But I look at things a little differently. In what kind of a world would two different expansions by Matthew and Luke be acceptable in the first place unless Mark expanded his gospel. The idea of an expanded gospel of Mark must have already been pre-existent. Just an observation. Not a big point. It's odd for a teacher to accept 3 different essays from 3 different students two of which are obvious expansions of a student named Mark.