Two Harmonic Waves Travelling in Opposite Directions Interfere to Produce

Multiple Choice

Two harmonic waves travelling in opposite directions interfere to produce a standing wave described by y = 2 sin (4x) cos (3t) where x is in m and t is in s. What is the speed (in m/s) of the interfering waves?

A) 0.75
B) 0.25
C) 1.3
D) 12
E) 3.0