Tom, clean up the broken glass _________ someone walks on it.
_____many times, he finally understood it.
Every year _____ money and energy is spent on environment protection.
I am not appealing for myself. I am appealing ________ all the ordinary teachers.
Nobody knows for sure what effect global warming has ___ our environment in the long term.
Witches(巫婆)are said to _______ magical powers to communicate with gods who are ______ of everything on the earth.