During an experiment a ringing alarm clock is placed inside a glass bell jar connected to a vacuum pump. As the air is gradually removed, the sound from the clock fades away even though the clock can still be seen vibrating. What conclusion is best supported by this observation?
(a) Sound travels faster in vacuum than in air.
(b) Light also needs a material medium to propagate.
(c) Sound requires a material medium for propagation.
(d) Sound can propagate through vacuum but its intensity becomes too low to hear.