托福 考试之2012托福写作考试必备抽象词
Periodic
用法：①ADJ. Periodic events or situations happen occasionally， at fairly regular intervals.
例句：They especially emphasized the need for natural， serene settings where hurried urban dwellers could periodically escape from the city. （2002-01）
例句：This periodic extinction might be due to intersection of the Earth” s orbit with a cloud of comets， but this theory
