Aline segment $AM=a$ moves in the $XOY$ plane such that $AM$ is parallel to the $x-axis$. If A moves along the circle $x^2+y^2=a^2$, then the locus of M is

Question

\[\begin {array} {1 1} (a)\;x^2+y^2=4a^2 & \quad (b)\;x^2+y^2=2ax \\ (c)\;x^2+y^2=2ay & \quad (d)\;x2+y^2=2ax+2ay \end {array}\]