Before revision: Dialogue systems in the open domain have achieved great success due to large conversation data and the development of deep learning, but  multi-turn scenarios are still a challenge because of the frequent coreference and information omission.
After revision: Dialogue systems in the open domain have achieved great success due to the easily obtained single-turn corpus and the development of deep learning, but multi-turn scenarios are still a challenge because of the frequent coreference and information omission.