Visitors have been asked to keep the woods clean and tidy.

问题描述:

Visitors have been asked to keep the woods clean and tidy.
请问中间为什么用have been asked

have been 表示过去完成时
ask sb. to do sth 是词组,让某人做某事,要求某人做某事
访问者/游客被要求保持森林整洁