Well, in an exam there's usually only one possible answer (otherwise it's a bad exam). There is always a clear hint in the question, so practice looking for that hint.
Speaking as a teacher, don't write both answers! I'd mark it as wrong because you clearly don't know which one is right. Some of my students write "?" next to their answers, and this really means nothing to the examiner.
And sorry for echoing the other members, but can you give us the question which confuses you?