"I mentioned the killings of journalists and I said that it is a matter of grave concern not just to the United States, but to the people of Russia, and not just to the activists, but to the people who worry that unsolved killings are a very serious challenge to order and to the fair functioning of society.

VOA: standard.2009.10.14

In these parts of the Politics Aristotle offers a serious challenge to existing Greek traditions and patterns of political education.

在这部份的《政治学》,亚里士多德对,当时的希腊政治教育传统与模式,提出严肃的挑战。

耶鲁公开课 - 政治哲学导论课程节选

"The EPRDF has discussed with parties that seem to approve or endorse its lines, not the serious parties that do challenge the EPRDF and do have serious issues about the political space in the country,"

VOA: standard.2009.11.01