occur的名词

例句：

When exactly did the incident occur?

这一事件究竟是什么时候发生的？

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。

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。

Nitrates are chemicals that occur naturally in water and the soil.

硝酸盐是水和土壤里天然存在的化学物质。