Question

The electric field associated with an e.m. wave in vacuum is given by →E=40cos(kz−6×10⁸t)^i, where E,z and t in volt/m, meter and seconds respectively. The value of wave vector k is
A 6m⁻¹
B 3m⁻¹
C 2m⁻¹
D 0.5m⁻¹