Before revision:  proposed a more precise classification method of MRC tasks with 4 different attributes  (2) we summarized 9 evaluation metrics of MRC tasks and (3) 7 attributes and 10 characteristics of MRC datasets;
After revision:  proposed a more precise classification method of MRC tasks with 4 different attributes (2) we summarized 9 evaluation metrics of MRC tasks , 7 attributes and 10 characteristics of MRC datasets;