Look at the picture above, which shows a small section of a "fossil" sand dune (a sand dune in which the grains have been "glued" together by hard-water deposits). when the dune was first deposited, which was up (which letter is closest to the arrow that is pointing in the direction you would have looked to see the sky when the dune was deposited)?
a., c
b., d
c., a
d., b