An alternative pipeline approach is to first solve P 2 and then P 1 , i.e., first determine whether there's a correct answer in the candidate set and then rank all candidates to find a correct one.
as we will show using state-of-theart Multiple Instance Learning (MIL) algorithms in Section 4, P 2 by itself is currently a very challenging task, partly because of the difficulty of extracting features from a set of candidate answers that are effective for answer triggering.