1)Police said the accident occurred about 4∶30 p.m．

2)This small tree also occurs in central and southern India．

3)The though of giving up never occurred to me．

occur为不及物动词，“to happen, especially unexpectedly”，如上面例句中的________；“come into sb's mind suddenly”，如上面例句中的________；“to exist or be found somewhere”，如上面例句中的________。