A pair of perpendicular straight lines passes through the origin and also through the point of intersection of the curve $x^2+y^2=4$ with $x+y=a$. The set containing the value of 'a' is

Question

$$\begin {array} {1 1} (1)\;\{-2,2 \} & \quad (2)\;\{-3,3\} \\ (3)\;\{-4,4\} & \quad (4)\;\{5,5\} \end {array}$$