Tom took a taxi to the airport, only ________ his plane high up in the sky.
I often go to the English corner to have a free talk with my classmates ____ I am busy.
In order to continue to learn by ourselves when we have left school, we must ________ learn how to study in the school now.
Unless they are programmed by humans, computers are nothing more than big pieces of metal. But once _____ what to do, they show extraordinary power in carrying it out.
—Mr. Smith, I wonder if I could take Friday off? — ______? It’s a bit difficult with so many people away on holiday, you know.
After five days’ climbing in the mountain, they reached ____ they thought was the place they’d been dreaming of.