William wants to borrow a book from the library. He comes to the library with his classmates. They can't see any assistant  in it, but only some robots standing there. He says to the robot, "Hey, give me a book on music." But the robot doesn't move. Then another student tells William, " You must say 'Excuse me' and 'please' first when you want some help." William does so and the robot brings the book. But William can't get the book from the robot's hands. He thinks for a moment, then he says a word to the robot. The robot gives him the book. John gets the book and goes home happily. Who works in the library?
Assistants Policemen Robots Teachers