Quantum Mechanics - Momentum Operator

The wave function of a particle is e^i(kx−ωt) where x is distance, t is time, and k and are ω positive real numbers. The x–component of the momentum of the particle is 

A. 0
B. ħω
C. ħk
D. ħω/c
E. ħk/ω 

(GR9277 #01)

Solution: 

Momentum operator, p = −iħ∇

pψ = −iħ ∂ψ/∂x 

= −iħ ∂e^i(kx−ωt)/∂x 

= ħk e^i(kx−ωt) 

= ħkψ  

Answer: C