请使用恰当的【一般过去时】或【过去进行时】A wood fire _______(burn) on the hearth, and a cat ______(sleep) in front of it. A girl _______(play) the piano and _______(sing) softly to herself. Suddenly there ______(be) a knock on the door. The girl_________(stop) playing. The cat _______(wake) up.