Storms most commonly occur_______a warm air mass and a cold air mass touch each other.
. They did everything they could to prevent the pollution ______ the river.
. Could I have a word with you, mum? — Oh, dear, if you ______.
. All but one _______ take part in the conference ______ tomorrow.
The fire was finally ______ after the building had ______ for two hours.
. Between the two rows of trees ____ the teaching building.