A pair of perpendicular lines passes through the origin and also through the points of intersection of the curve $x^2+y^2 =4$ with $x+y=a,$ where $a > 0$. Then $a=$

Question

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