问题补充：
When the teacher came into the classroom, the students in groups.
A. work
B. have worked
C. were working
D. are working
答案：
A. work
B. have worked
(答案→)C. were working
D. are working
解析：句意：当老师走进教室的时候，学生正在分组学习。老师进来这个动作发生在过去，所以学生正在做某事，是过去的动作，所以用过去进行时态，故选C
