The Fact That All Dogs in the Neighborhood Begin to Howl

Multiple Choice

 The fact that all dogs in the neighborhood begin to howl at the same time every day while you can't hear anything that might cause this response is probably because the dogs

A) can hear sounds we would classify as infrasound.
B) can hear sounds we would classify as ultrasound.
C) are disturbed by underground vibrations.
D) require more volume before they can hear the same sounds as you do.