For a transverse wave travelling along a stretched string, which one of the following statements is correct at any instant?
(a) The speed of the wave equals the speed of every particle of the string at all times.
(b) Whenever the particle speed becomes zero, the wave momentarily stops moving.
(c) Particles can momentarily have zero speed while the wave continues to travel at constant speed.
(d) The speed of the wave decreases when the displacement amplitude of the particle decreases.