20. When Jack arrived, he learned Mary ____ for almost an hour.
—What's the boys' favorite sport?—__________ of boys like playing football.
Every possible means __________ to prevent air pollution, but the sky is still not clear.
We must __________ to the public the simple fact thatdrugs are dangerous.
That was the first time __________ I __________ face to face with the president.
I walked up to the top of the hill with my friend, __________ we enjoyed a splendid view of the lake.