For a perfect correlation between the variables $x$ and $y$ the line of regression is $ ax+by+c=0$ where $a,b,c > 0$ then $ r$ is

Question

$\begin {array} {1 1} (A)\;0 & \quad (B)\;-1 \\ (C)\;1 & \quad (D)\;None \: of \: these \end {array}$