1 Introduction  
2 Adversarial Example Generation in Question Answering Systems  
2.1 Overview of Question Answering Systems  
2.2 Existing Approaches of Adversarial Example Generation in Question Answering Systems  
2.3 Discussion  
3 Multimodal Question Answering Systems  
3.1 Overview of Multimodal Question Answering Systems  
3.2 Existing Approaches of Multimodal Question Generation  
3.3 Existing Approaches of Adversarial Example Generation in Multimodal Question Answering  
3.4 Discussion  
4 Defense Mechanisms and Evaluation  
4.1 Defense Mechanisms  
4.2 Adversarial Datasets  
4.3 Evaluation Methodologies  
5 Discussion of Future Research Directions