They were surprised when they heard the whisper.They were surprised ----- ----- the whisper.

问题描述:

They were surprised when they heard the whisper.They were surprised ----- ----- the whisper.

They were surprised when they heard the whisper.They were surprised when hearing- the whisper.
状语从句和非谓语之间的转换
主语they是主动听,所以后面动词用的是现在分词when hearing