It can happen suddenly or over a period of time.

它可以发生突然或以上的在一段时间内。

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Weaning occurs gradually, and the strikes happen suddenly.

断奶是一个渐进的过程，而罢奶是突然发生的。

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Are the changes gradual or do they seem to happen suddenly?

这些变化是逐步发生的还是突然出现的？

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Slow earthquakes are different from violent earthquakes, which happen suddenly and can be extremely destructive.

VOA: special.2009.07.07

Just a couple of months ago London seemed to be soaring and now suddenly there's this pessimistic atmosphere because we don't know what's going to happen in London.

仅仅数月之前,伦敦房价还一路高歌,忽然之间市场一片愁云惨淡,我们不知道伦敦的未来状况

耶鲁公开课 - 金融市场课程节选

The city state's transport system would benefit from being better able to match the demand and supply of taxis, particularly when it rains, which tends to happen suddenly in Singapore.

ECONOMIST: Urban research

Cholera outbreaks, such as the one that hit Peru and its neighbours in the early 1990s, tend to happen suddenly, as the formerly benign bacteria turn on their pathogenic apparatus when they hit the body.

ECONOMIST: The cholera genome

Ask most computer programmers what would happen if, suddenly, their computers got a thousand times faster.

FORBES: Magazine Article