Final answer:
Beatty threatens to shoot the Mechanical Hound after picking up the green bullet in his ear.
Step-by-step explanation:
In the novel Fahrenheit 451, Beatty, the captain of the firemen, threatens to "shoot the Mechanical Hound" after he picks up the "green bullet" that was in his ear. The green bullet is a listening device used by the government to monitor conversations. Beatty's threat indicates that he is suspicious of being monitored and is willing to take drastic measures to protect himself.