英语 根据下面各题中划线的情态动词,判断说话人对句中动作 (14 21:12:38)

比较：
a:Students ought to listen to their teacher carefully in class.
b:Students must listen to their teacher carefully in class.
c:Students have to listen to their teacher carefully in class.
a句表示学生在上课时有仔细听课的义务,不含强迫之意.
b句表示学生必须在上课时仔细听课,含主观强制之意.
c句表示按照规章制度,学生在上课时有仔细听课的义务,含有外界强制之意.