When montag returns to the firehouse to come back to work and turn in the book, what is missing? gradesaver?

Question:

When Montag returns to the firehouse to come back to work and turn in the book, what is missing?

The Robotic Dog or Mechanical Hound is Missing

Explanation

Fahrenheit 451 is a 1953 novel by Ray Bradbury denoting an imagined state or society where there is great suffering. He depicts a future world as one where the where television is the primary form of entertainment and books are banned as subversive.

So in this world, Montag is a fireman whose responsibility is to incinerate books, rather than fire-fighting. At the fire station, there is a Robotic Dog, whose primary use was the smoking out of book owners who in that time were regarded as fugitives. At the beginning the Hound seems suspicious of Montag, and threatens him; later, when Montag flees, another Hound gives him a hot chase which is broadcast on live TV for citizens entertainment.

During Montag's crisis of faith in the system, he returns to the firehouse in the middle of the night. On getting there he realises that:

The Mechanical Hound was gone and Montag came in through the silence and touched the brass pole and slid up in the dark air, looking back at the deserted kennel, his heart beating, pausing, beating.

The Mechanical dog disappeared  because Chief Beatty has sent it to Montag's house, to wait for them. Chief Beatty knows that Montag has been reading and keeping books. His plan was to use Montag's fear of the dog to make him renounce his curiosity. In the end, Montag destroys the dod and flees.  

Cheers

what are the answer choices for this question