In Which of the Following Circumstances Would Police Officers Be

In which of the following circumstances would police officers be required to "knock and announce" their presence before executing a search warrant?

A) Officers are told by an informant that a home's occupant plans to shoot any officer that comes to their door.
B) Officers are aware that the occupant of the home is armed and dangerous.
C) Officers have a warrant to search for drugs, but when coming up to the residence, they hear the occupants say, "Quick, flush it down the toilet!" followed by running footsteps.
D) Officers have a warrant to search for drugs, but the occupants of the house do not appear to be home.