O-6：once in a blue moon 极为罕见

　　A:The river is much higher than the ground.

　　A:河比地高得多。

　　B:How dangerous!

　　B:多危险呀!

　　A:Yes, it is.

　　A:是呀。

　　B:What if the river overflowed its bank?

　　B:河水要是涨过了岸，怎么办?

　　A:That would be disastrous.

　　A:那可遭殃了。

　　B:The whole city will get flooded?

　　B：整个城市就会被水淹了?

　　A:Of course. But that happens only once in a blue moon.

　　A:就是，不过那百年也不见得会遇上一次。