The electric field associated with an e. m. wave in vacuum is given by $\overrightarrow{E}=\hat{i}40 cos(KZ-6\times10^8t)$,where E, z and t are in ${volt}{m}$, meter and seconds respectively. The value of wave vector k is :

Question

( A ) $0.5 m^{–1}$
( B ) $3 m^{–1}$
( C ) $2 m^{–1}$
( D ) $6 m^{–1}$