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。
