It refers to a time-frame leading up to the moment of speaking, so the past perfect refers to a time-frame leading up to a point in the past;

It refers to a time-frame that is in some way separated from the present; there is a break between the completion of the event and the present moment;

It suggests that the events may be seen more as background or of secondary importance;

It is often used for events which started in the past and are still continuing, or which have stopped, but whose effects are still continuing;

None of the answers.