蝴蝶的生命周期研究起来很有趣。

The life ___cycle of a butterfly is fascinating to study.

许多自然过程，如水循环，对地球上的生命至关重要。

Many natural processes, like the water ___cycle, are essential for life on Earth.

繁荣与萧条是许多国家经济中常见的循环周期。

Boom and bust is a common ___cycle in the economics of many countries.