They raised a flag.

他们升起一面旗子。

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They raised a national flag.

他们升起了一面国旗。

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The presentations themselves should have raised a red flag for careful users, we think, but we have no way of knowing how successful they were at this time.

我们想细心的用户可能会发现这些幻灯片的危险之处，但是我们没办法知道目前用户成功识别的几率如何。

youdao

They raised the pirate flag -- with its picture of a skull and crossed bones.

VOA: special.2009.05.24

But several admissions officers interviewed said they occasionally Google students to learn more about a project in which they were involved, or because a red flag was raised in an interview, recommendation or somewhere else.

WSJ: Web Profiles Haunt Students

At the end of this week, Ed Miliband is safe in his job, he has shown he can rise to the occasion, he has raised morale in his party and they leave with a spring in their step as they hum the Red Flag (most don't know the words these days) and make their way home.

BBC: Where do Labour stand after 2012 conference?

Companies across industries have raised a red flag, noting that rising costs will begin to eat into their earnings and that they will be forced to raise prices.

FORBES: PepsiCo, Starbucks And Others Wave Red Flag Of Inflation