I know that saying:
(A) I am here since six o’clock. or (B) I was here since six o’clock.
would sound unidiomatic because of using "since" and its requisite tense which is present perfect. For example:
(C) - I have been here since six o’clock.
But I came across another sentence somewhere in a textbook which caused me to doubt. Is it possible to say:
(D) - I had been there since six o’clock.
If yes, what's the semantic difference between "C" and "D", and when should I use each one?