To create training data for unanswerable question generation, we use (plausible) answer spans in paragraphs as pivots to align pairs of answerable questions and unanswerable questions. In this way, we obtain the data with which the models can learn to ask unanswerable questions by editing answerable ones with word exchanges, negations, etc. 
 Question: How do they ensure the generated questions are unanswerable?
learn to ask unanswerable questions by editing answerable ones with word exchanges, negations, etc