--Why are the windows so dirty?--Because we_________them for a long time.A. have cleaned B. haven't cleaned C. didn't clean
—Sir,could you please put out your cigarette?This is a smoke-free(无烟的)school. —________.
Few people think about what they can do________others.
—What are they doing now? —They are busy________articles in the room.
They decided________the homeless kids.
There are many trees on________side of the road.