Find the area of the triangle formed by the lines $y – x = \theta, x + y = \theta$ and $x – k = \theta$

Question

$\begin {array} {1 1} (A)\;k\: sq\: units & \quad (B)\;2k\: sq\: units \\ (C)\;k^2\: sq\: units & \quad (D)\;2k^2\: sq\: units \end {array}$