问题补充：
The students pretended attentively when the teacher came in.
A. to learn
B. learning
C. and learned
D. to be learning
答案：
A. to learn
B. learning
C. and learned
(答案→)D. to be learning
解析：考察不定式的进行时,pretend to be doing,假装正在做什么,句子的意思是学生们在老师进来的时候假装正在认真学习，故选择D
