Difficulties strengthen the mind, ______labor does the body.
Willpower is a kind of quality, _____ is _____ it takes to do anything well.
Tom is keen _____ surfing the Internet in his spare time.
The students were completely _____ in their reading.
She missed the plane, _____ driving very fast to the airport.
My brother is really _____. He often works in his office far into the night.