When I am alone, I hear the footsteps of my friend in my imagination:

(A) The footsteps of my friend heard by me, when I
am alone.
(B) The footsteps of my friend is heard by me when I am alone.
(C) The footsteps of my friend are heard by me when I am alone.
(D) The footsteps of my friend was heard by me when I am alone.