Complete the following sentences :
(a) Sound can not travel through ___, but it requires a ___ .
(b) When sound travels in a medium, the particles of medium ___ but the disturbance ___ .
(c) A longitudinal wave is composed of compression and ___ .
(d) A transverse wave is composed of crest and ___ .
(e) Wave velocity = ___ x wavelength.
Explain the terms crest and trough in relation to a transverse wave.