A. unusual
B. front
C. upper
D. back
rear : the back part of something, especially a building or vehicle.
unusual : not habitually or commonly occurring or done.
front : the side or part of an object that presents itself to view or that is normally seen or used first; the most forward part of something.
upper : situated above another part.
back : the side or part of something that is away from the spectator or from the direction in which it moves or faces.
Antonym of rear is front
