  This paper presents an extension of the Stochastic Answer Network (SAN), one
of the state-of-the-art machine reading comprehension models, to be able to
judge whether a question is unanswerable or not. The extended SAN contains two
components: a span detector and a binary classifier for judging whether the
question is unanswerable, and both components are jointly optimized.
Experiments show that SAN achieves the results competitive to the
state-of-the-art on Stanford Question Answering Dataset (SQuAD) 2.0. To
facilitate the research on this field, we release our code:
https://github.com/kevinduh/san_mrc.
