Why Do Actors and Observers Tend to Give Different Explanations

Why do actors and observers tend to give different explanations for the same instance of behaviour by an actor

A)  Only outside observers can accurately explain actors' behaviour. 
B)  Situational pressures may not be readily apparent to an observer. 
C)  Observers tend to possess more knowledge than the actors. 
D)  Only actors themselves can accurately explain their own behaviour.