Seeing their teacher into the classroom they stopped at once.A. walk; tellingB. enterin

问题补充：

Seeing their teacher into the classroom, they stopped at once.

A. walk; telling

B. entering; to speak

C. enter; to tell

D. walking; talking

答案：

A. walk; telling

B. entering; to speak

C. enter; to tell

(答案→)D. walking; talking

解析：句意：看见他们的老师走进教室，他们立即停了下来。现在分词作宾语补足语表示正在进行的伴随性动作，和被修饰词是主谓关系。Stop后接动名词表示停止正在做的动作，接不定式表示停下来去做另一件事情，结合语境可知选D。