what would be the expression in place of ? in the Van't Hoff equation: $log \frac{K_2}{K_1} = \frac {\Delta H^\circ}{2.303R} [?]$

Question

(a) $\frac {1}{T_1} + \frac {1}{T_2}$

(b) $\frac {1}{T_1} - \frac {1}{T_2}$

(c) $\frac {1}{T_2} - \frac {1}{T_1}$

(d) $\frac {1}{T_1 T_2}$