15. --- Who are the teenagers gathering at the school gate? --- Well, if you _____ know, they are all my visiting students.
—Why not take this road? It’s a short cut. — I am afraid we can’t. It _____.
__________ he is old, the old man still insists on studying at a night school.
__________! Let’s go there and have a look at it.
My father bought me _____ mp3, but I lost _____ mp3 yesterday.
_______the room,I found my cat lying on the floor,dying.