Two sinusoidal plane waves of the same frequency having intensities $I_{e}$ and $4 I_{e}$ are travelling in the same direction. The resultant intensity at a point at which waves meet with a phase difference of zero radian is
(a) $I_{\text {o }}$
(b) $5 I$
(c) $9 I$
(d) $3 I_{o}$