托福考试：托福写作考试必备抽象词

免费留学评估 免费移民评估 免费签证评估 免费美国留学评估 免费英国留学评估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 is purely speculative. （2002-08）例句：Periodically， molten material wells out of the Earth’s interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. （1999-01）例句：The woolly mammoth， along- haired rhinoceros， and other mammals have been periodically exposed in the tundra of Siberia， the hair and red flesh still frozen in cold storage.（1997-12）

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