http://i.min.us/ibQyJw.JPGMy mistake, I was talking about opening the epoxide under acidic conditions with H2O as a nucleophile.
Product B is a syn diol (both OH are facing up) and I have never heard my instructor or text book talk about an epoxide opening with H3O+ give a syn diol. The method to make syn diols I learnt was from alkenes using OsO4 followed by H2O.
Based on that I think the mechanism for epoxide opening
doesn't involve a planar carbocation where the nucleophile can attack from top and bottom resulting in product A and B. Rather, I think it's SN2 attack by the nucleophile from below the epoxide giving trans product A. Product B is not formed because the nucleophile cannot attack from the top (the epoxide is shielding the top side).
If we use the same concept for ethanol as nucleophile, then I am guessing Product A is the only possible product. My instructor didn't go into much detail regarding stereochem and that is confusing part.