In a flask colourless $N_2O_4$ is in equilibrium with brown coloured $NO_2$. At equilibrium when the flask is heated at $100^\circ C$, the brown colour deepens and on cooling it becomes less coloured. The changed in enthalpy, $\Delta H$ for this system is

Question

(a) Negative
(b) Positive
(c) Zero
(d) Undefined