My tongue rests at the top, lightly touching my palette. As far as I am aware, this is the optimum position. (Or so it appears from studying books on meditation and yoga.)
When my mouth is open, my tongue rests on the bottom. When my mouth is closed completely, then it is on the top. Really, it's on the top and bottom when my mouth is closed, because it kinda fills up all that space.
So my answer is "Both A and B"