Some areas, _____ their severe weather conditions, are hardly populated.
If _____ carefully, the experiment will be successful.
He is a man of few words, and seldom speaks until _____ to.
They kept trying _____ they must have known it was hopeless.
There are five pairs, but I'm at a loss which _____ to buy.
He was not an actor, who often appeared on stage, _____ a writer, writing stories.