A third presidential candidate emerged to mount a serious challenge and throw the campaign wide open.

第3位总统候选人上场郑重发出挑战，令这次竞选充满悬念。

《柯林斯英汉双解大词典》

And I think this is a serious challenge.

我认为这是一个严重的挑战。

youdao

Mobile devices present a serious challenge.

移动设备面临巨大挑战。

youdao

Even if U.S.economic output expands in 2010 and more Americans head back to work, both critics and supporters of the president say the nation faces a more serious challenge to future economic viability: a rapidly growing deficit and a national debt estimated at more than $12 trillion.

VOA: standard.2010.01.15

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

"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

Carroll has not mounted a serious challenge and Akaka is likely to win this contest.

CNN: Both parties vie for control of the Senate

Neither it nor the two small parties outside the coalition can yet offer a serious challenge.

ECONOMIST: Poland��s progress

So the number of Conservative-run authorities planning council tax increases represents a serious challenge.

BBC: More local authorities look set to increase council tax