Why did German U-boats not warn their targets before sinking…

Why did Germаn U-bоаts nоt wаrn their targets befоre sinking them, as outlined in the standard rules of warfare? a.    Because they were dirty rotten bastards. b.    Because U-boats can only fire while underwater. c.    Because the British urged merchant ships to install guns and fire on or ram U-boats. d.    Because the official rules of warfare were changed in an international conference.