I remember that BC question! Man that was a tough one, and I picked the rectangle like you did. I tried to figure out a way to justify it when I reviewed my mistakes at the end but I couldn't.
Edit:
Now that I think of it, if it was a rectangle, that line in between the two would be signaling a change in elevation from the front view right? So in the end view, we would have seen this difference if it was a rectangle. Also answer choice D is signaling a change in elevation from the top on the rectangle portion, but that isn't seen in what views are given to us.