When these side effects occur, they are called adverse reactions.(当这些副作用发生时，它们被称为不良反应。)
Rattlesnakes occur in the warmer, drier parts of North America.(响尾蛇出现在北美温暖干燥的地区。)
When exactly did the incident occur?(这一事件究竟是什么时候发生的？)
Does it look like something that might occur in nature?(它看起来像自然界中可能发生的事情吗？)
Most volcanoes occur at plate boundaries.(大多数火山的爆发发生在板块边界。)
If Dollo's law is right, atavisms should occur only very rarely.(如果多洛定律是正确的，返祖现象应该很少发生。)
These snails do not occur on low-lying coral islands.(这些蜗牛不会出现在低洼的珊瑚岛上。)
They're more likely to occur in societies that frown on divorce.(这种关系更可能存在于反对离婚的社会中。)
Additional breathing changes occur during REM sleep that are even more dramatic than the changes that occur during NREM.(在快速眼动睡眠期发生的额外的呼吸变化，甚至比那些在非快速眼动期发生的变化还要显著。)
Such events occur only when the external conditions are favourable.(此类事件只有当外界条件有利时才会发生。)
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