问题
选择题
The students _______ their homework carefully when the teacher walked into the classroom quietly.
A．do
B．did
C．are doing
D．were doing
答案
答案：D
题目分析：句意：学生们正在认真做作业，当老师安静地走进教室时。根据when the teacher walked into the classroom quietly当老师安静地走进教室时，可知学生们正在做某事，从句为一般过去时，主句为过去进行时，故选D.