In a Young's Double-Slit Experiment, the Time-Averaged Value Of $\cos ^ { 2 } \left( \omega t - 2 \pi r _ { 0 } / \lambda \right)$

Multiple Choice

In a Young's double-slit experiment, the time-averaged value of $\cos ^ { 2 } \left( \omega t - 2 \pi r _ { 0 } / \lambda \right)$ , where $r _ { 0 }$ is the distance from the center of the slits to the point of interest, and $\lambda$ is the wavelength of light, is

A) 0
B) 1/2.
C) $r _ { 0 } / \lambda .$
D) $1 / \sqrt { 2 }$