The exam was very difficult but everyone ______ pass.

晚饭后，你散步时遇到你的一个朋友，你应说_________ 来打招呼。

There were few doctors, _____ he had to work very hard _____ his own.

It never occurred to me _____ you could succeed in persuading him to change his mind.

Only when the villagers were faced with water shortage___ how serious the problem of water pollution was．