It ___ be the postman at the door. It’s only six o’clock.
Mrs. Johnson, our neighbor, often ______ the dog in the park after supper.
They soon ______ doing the same thing day after day, and finally gave up.
I like to listen to music ______ I’m having a walk in the evening.
______ the terrible pollution, so many beautiful fish are fast disappearing in the river.
All of us found ______ difficult to work out the maths problem.