The Longitudinal Displacement of a Mass Element in a Medium

Multiple Choice

The longitudinal displacement of a mass element in a medium as a sound wave passes through it is given by s = s_m cos (kx - ωt) .Consider a sound wave of frequency 440 Hz and wavelength 0.75m.If s_m = 12 µm, how long does it take an element of air to move from a displacement of 12 µm to a displacement of 0 µm?

A) 0.57 ms
B) 1.1 ms
C) 2.3 ms
D) 3.4 ms
E) 4.6 ms