the class the students left the classroom one by one.A. At the end of B. In the end C.

问题补充：

the class, the students left the classroom one by one.

A. At the end of B. In the end C. By the end

答案：

解析：根据语境可知此句的含义是课末，学生们一个接一个的离开了教室。A表示在…末，B表示最后相当于at last；C没有这种表达方式，by the end of表示到…末，根据题干的含义可知选择A。