This is a question on simple harmonic motion and resonance?

A tuning fork of natural frequency 264 Hz sits on a table at the front of a room. At the back of the room, two tuning forks, one of natural frequency 260 Hz and one of 420 Hz, and initially silent; but when the tuning fork at the front of the room is set into vibration, the 260-Hz fork spontaneously begins to vibrate, but the 420-Hz fork does not. Explain why this happens?