A ringing electric bell is suspended inside an airtight glass jar connected to a vacuum pump. As air is gradually removed, the sound becomes faint and finally inaudible although the bell is still seen to vibrate. The most reasonable conclusion is that
(a) vacuum absorbs all the sound energy produced by the bell
(b) removal of air lowers the frequency of the bell below the audible range of the human ear
(c) sound cannot propagate without a material medium
(d) removing air makes the bell vibrate with a very small amplitude, so the ear cannot hear it