If $dx+dy=(x+y)(dx-dy)$ then $\log (x+y)$ is equal to:

Question

\[\begin {array} {1 1} (a)\;x+y+c & \quad (b)\;x+2y+c \\ (c)\;x-y+c & \quad (d)\;2x+y+c \end {array}\]

(In the above , c denotes a constant )