Officer Goldman Responds to a Call of an Active Shooter

Multiple Choice

Officer Goldman responds to a call of an active shooter during a home invasion at a local residence.  Officer Goldman arrives on the scene and quickly surveys the residence.  He and his partner immediately enter the home and see the suspect.  They command the suspect to drop his weapon, and the suspect responds by dropping the weapon.  Officer Goldman arrests the suspect and searches him.
Officer Goldman immediately entered the home without obtaining a warrant.  He was allowed to do this based on the requirement of

A) consent.
B) exigent circumstances.
C) arrest.
D) seizure.