David fee studies sound waves so deep and low that humans cannot hear them. what are these low frequencies
called?
infrared
seismology
geosound
infrasound

infrasound

Explanation:

The infrasound is called as the low frequency sound. The frequency of these sounds is less than 20 Hz. The sound waves of the infrasound cannot be perceived by the humans as human ear are less sensitive to the low frequency. The infrasound can be produced in extreme weather conditions, earthquakes, meteor impacts, explosive detonation, collision of oceanic waves, waves on beach, and sound produced by the air conditioner.

"a.) well–developed vascular system."

that is wrong, its not thymus.