Mr. Smith has two cats. One is big and the other is small. He likes them very much. One day his good friend Mr. White comes to see him. He is very surprised .He finds there are two holes  in the door, a big hole and a small hole. He says, "My dear friend, why are there two holes in your door?" "To let them in and out." Mr. Black answers. "But why are there two holes?" his friend asks, "How can the big cat go through the small hole?" According to this passage.   _   is not necessary .
The big hole The small hole The big door. The small door.