Why does Caesar say "Et tu, Brute"?

A. Caesar says "you too, Brutus" because he is concerned that Brutus is participating in bullying a lesser known Roman.

B.Caesar says "you too, Brutus" because he is surprised to see Brutus with the Soothsayer.

C. Caesar says "you too, Brutus" because he is saddened that Brutus did not trust Antony.

D. Caesar says "you too, Brutus" because he is surprised to see Brutus with the conspirators on Caesar's deathbed.