Suddenly, a red warning light turned on.

突然，一个红色警告灯亮了。

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This example demonstrates turning on a red warning light if the build fails.

这个例子演示了在构建失败时，打开一盏红色的警示灯。

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Imagine that you are driving in your car and the red warning light comes on.

假如你在开车的时候，红色警示灯突然亮了起来。

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The red Fire Warning light is bad news when it lights up, but you can silence the alarm bell by pressing it.

FORBES: What Do All The Controls In An Airplane Cockpit Do?

Last month the Commons Education Select Committee issued a damning report into the plans, which it described as a "red light" warning over the changes.

BBC: Q&A: Changes to the teaching of key subjects

Last week a report from the Commons Education Select Committee issued a "red light" warning to the government, urging it to slow down and rethink its proposed changes to GCSEs and the exam system.

BBC: Planned switch from GCSEs to Baccalaureate in England 'abandoned' - BBC News