the sound on the step the boy rushed out to see if his mother had come back.A. To hearB.

问题补充：

the sound on the step, the boy rushed out to see if his mother had come back.

A. To hear

B. Hearing

C. Heard

D. Hear

答案：

A. To hear

(答案→)B. Hearing

C. Heard

D. Hear

解析：考查现在分词做状语，the boy和hear是主谓关系，用现在分词做伴随状语，句意：听到台阶上的声音，男孩冲出去看看是不是妈妈回来了。选B。