The abnormal bleeding is your body's own red flag of danger.

这反常的流血是你身体本身的危险信号。

《柯林斯英汉双解大词典》

The national flag of China is red.

中国的国旗是红色的。

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One of the most famous places is the Red Flag Canal in Linzhou which was built on the side of cliffs of the Taihang Mountains.

最有名的地点之一是河南林州的红旗渠，它建在太行山的悬崖边上。

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Iraqi President Jalal Talibani is getting the red-carpet treatment in France, with the Iraqi flag flying on the Champs Elysees in Paris.

VOA: standard.2009.11.16

But this propeller-driven aircraft is bright white with a black iron cross and the red,yellow,and black flag of the German Luftwaffe on its side.

VOA: standard.2010.07.27

The region is home to the Red Flag Canal, a massive water project.

ECONOMIST: Farmers pay the price for decades of wasteful water use

The biggest red flag is loud, persistent snoring, says Haviva Veler, director of the Weill Cornell Pediatric Sleep Center in New York, which opened in 2012.

WSJ: More Children With Sleep Problems Seek Overnight Tests

But perhaps the biggest red flag is the potential for conflict of interest, as a small number of researchers look to start up what could be a big new research field for academics and industry alike.

FORBES: BP's Biotech Bet