Unless carbon emissions are brought under control,he says, the challenge of adapting to global climate change will become ever more difficult.

VOA: standard.2010.06.14

And scientists have been working on this for years and there's been progress, but it's still a very difficult challenge.

科学家们对此已经研究很久了,并取得了进展,但仍有很大挑战

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

That's a very difficult challenge in the Senate right now. It's not looking very good these days,

目前这在参议院是很大的挑战。现今的形势看上去不容乐观。

对全球变暖的政策 - SpeakingMax英语口语达人

The national security challenge comes at a difficult time for Mr.Obama, who later this month will mark his first year in office.

VOA: standard.2010.01.06

So in retrospect, I think the way to deal with crisis, deal with challenge, deal with difficult times, is like this: First, you need to be realistic.

回首去年，我们应对危机，面对挑战时，所采取的方法就是：,首先，要变得现实一些。

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So I think that it's true that the economy is difficult and there is big challenge in front of us.

所以我觉得，目前的经济形势真的很困难，我们面临着很大的挑战。

事业就是挑战 - SpeakingMax英语口语达人