Given below are some functions of x and t to represent the displacement (transverse or longitudinal) of an elastic wave. State which of these represent (i) a travelling wave, (ii) a stationary wave or (iii) none at all:
(a) y = 2 cos(3x) sin(10t)
(b) y = 2√{x - vt}
(c) y = 3 sin (5x - 0.5t) + 4 cos (5x - 0.5t)
(d) yt = cos x sin t + cos 2x sin 2t
