Declarations,commitments and speeches don't feed hungry people, He says the scale of the challenge is significant. There are more than one billion poor and hungry people in the world today.

VOA: standard.2010.03.29

But the challenge is that we really didn't have a business model at that time.

但问题是,那时公司没有一个商业模式。

斯坦福公开课 - 百度CEO李彦宏演讲：全球最大搜索引擎的发展课程节选

But you know, I understand it's a challenge when you're coming from a language that doesn't have any concept of an article.

但是，我能够理解当你的母语没有冠词的概念时，它将是一个挑战。

经常犯的失误 - SpeakingMax英语口语达人

Not a week goes by when he doesn't challenge the other partners to slow down.

FORBES: The wild bunch

If we don't challenge them, how will we ever know that they can't do it?

CNN: STORY HIGHLIGHTS

She doesn't challenge the teachers because, she says, people say all travellers do is argue and fight.

BBC: Anonymous teenager