＿＿＿,the students put away their books,rose from their seats and left the classroom

题目

＿＿＿,the students put away their books,rose from their seats and left the classroom
A.As class being over
B.Class being over
choose B .why?

答案

class being over,现在分词表伴随状态.
如果是A,要改为,as class was over,因为as后接句子.