Mr. Irvine, who ________ in a high school for forty years, is now living in retirement.
In that month, he earned as much as, if ____ than, $40 000.
The research is so designed that once _____ nothing can be done to change it.
If _____ carefully, the experiment will be successful.
He is a man of few words, and seldom speaks until _____ to.
There are five pairs, but I'm at a loss which _____ to buy.