例句：
When exactly did the incident occur?
這一事件究竟是什么時候發(fā)生的.？
These snails do not occur on low-lying coral islands.
這些蝸牛不會出現(xiàn)在低洼的珊瑚島上。
Rattlesnakes occur in the warmer, drier parts of North America.
響尾蛇出現(xiàn)在北美溫暖干燥的地區(qū)。
Nitrates are chemicals that occur naturally in water and the soil.
硝酸鹽是水和土壤里天然存在的化學物質(zhì)。