If $\sigma_x= \sigma_y$ and $x,y$ are related by $u=x+y\: v=x-y$ then $cov (u,v)$ is

Question

$\begin {array} {1 1} (A)\;-1 & \quad (B)\;0 \\ (C)\;1 & \quad (D)\;cov\: (x,y) \end {array}$