Flag question if $a_t$ is a white noise $(0,4)$ Then, the auto$-$covariance function of lag Zero for the process $x_t=0\mathrm{.}2x_{t-1}+0\mathrm{.}35x_{t-2}+a_t$ is: Select one: a$.0\mathrm{.}35$ b$.4$ c$.0\mathrm{.}2$ d$.5\mathrm{.}0351$