The objective function $Z = ax + by$ of an LPP has maximum value $42$ at $(4, 6)$ and minimum value $19$ at $(3, 2)$. Which of the following is true ? (A) $a = 9, b = 1$ (B) $a = 5, b = 2$ (C) $a = 3, b = 5$ (D) $a = 5, b = 3$