OBVIOUSNESS
(as) plain as the nose on one's face
obvious; clearly evident. (Informal.)What do you mean you don't understand? It's as plain as the nose on your face.Your guilt is plain as the nose on your face.
goes without saying
[something] is so obvious that it need not be said.It goes without saying that you are to wear formal clothing to dinner each evening.Of course. That goes without saying.
like a sitting duck and like sitting ducks
unguarded; unsuspecting and unaware.He was waiting there like a sitting duck—a perfect target for a mugger.The soldiers were standing at the top of the hill like sitting ducks. It's a wonder they weren't all killed.