The corner points of the feasible region of a Linear Programming Problem are $(0, 2), (3, 0), (6, 0), (6, 8)$ and $(0, 5)$. If $Z = ax + by; (a, b > 0)$ be the objective function, and maximum value of $Z$ is obtained at $(0, 2)$ and $(3, 0)$, then the relation between $a$ and $b$ is : (A) $a = b$ (B) $a = 3b$ (C) $b = 6a$ (D) $3a = 2b$