happenvi.发生;碰巧;出现;偶遇
Happening once in four years.每四年发生一次的
Happening very rapidly or easily来得快的;来得容易的
happen in a particular manner.以一种特定的方式发生。
Happening by accident or chance.偶然发生的因某种事件或机会而发生
Happening once every five years.每五年发生一次的