"The biggest challenge of getting life saving vaccines to poor countries is that there's no market.

VOA: special.2010.03.29

So we use his works as a challenge to say "No".

所以我们把他的成就当作一种挑战，说“不”。

以爱因斯坦为榜样 - SpeakingMax英语口语达人

There are no books we can salvage. That would be another quite challenge.

VOA: standard.2009.10.06

Ndoungou told reporters outside the court that concerning the electoral list and the comportment of electoral officials during the process, he sees no call for further action by the court to challenge the outcome.

VOA: standard.2009.10.04